one_gadget

OneGadget

When solving CTF pwn challenges we usually want a one-gadget RCE: a single address in libc that, jumped to, calls execve("/bin/sh", NULL, NULL).

This gem finds them for you, so you don't have to dig through objdump or IDA Pro every time 😉

Point it at a libc -- one_gadget /path/to/libc -- and enjoy the magic 😆

Installation

Available on RubyGems.org!

$ gem install one_gadget

Supported Architectures

[x] i386
[x] amd64 (x86-64)
[x] aarch64 (ARMv8)
[x] arm (ARMv7, A32/Thumb-2)
[x] riscv64 (RV64GC)
[x] mips (MIPS32 o32, big- and little-endian)

Implementation

OneGadget symbolically executes the code around each candidate address to work out what has to hold for the gadget to work.

Gadgets are found by walking the control-flow graph backward from each exec/posix_spawn call, following conditional branches both ways. When a gadget is only reachable if a branch is (not) taken, that decision shows up as an extra constraint, e.g. x2 == 0x1.

The article introducing how I develop this tool can be found on my blog.

Checked by running them

Constraints are only worth as much as they are complete, so the gadgets this repository ships are verified by being run. Aletheia loads the libc in a live process, arranges exactly what a gadget's constraints ask for, poisons every register and page they don't mention, jumps to the offset, and requires a real /bin/sh to come back and list the root directory. A gadget whose constraint list is missing something fails there instead of passing by luck -- which is how several missing constraints were found.

bundle exec rake aletheia:verify                # every libc under spec/data, level 0
bundle exec rake "aletheia:verify[1, aarch64]"  # one output level, one architecture

Aletheia is development tooling: it lives outside lib/ and bin/, so it is not part of the published gem.

Usage

Command Line Interface

$ one_gadget
# Usage: one_gadget <FILE|-b BuildID> [options]
#     -b, --build-id BuildID           BuildID[sha1] of libc.
#     -f, --[no-]force-file            Force search gadgets in file instead of build id first.
#     -l, --level OUTPUT_LEVEL         The output level.
#                                      OneGadget automatically selects gadgets with higher successful probability.
#                                      Increase this level to ask OneGadget show more gadgets it found.
#                                      Default: 0
#     -n, --near FUNCTIONS/FILE        Order gadgets by their distance to the given functions or to the GOT functions of the given file.
#     -o, --output-format FORMAT       Output format. FORMAT should be one of <pretty|raw|json>.
#                                      Default: pretty
#     -r, --raw                        Alias of -o raw. Output gadgets offset only, split with one space.
#     -s, --script exploit-script      Run exploit script with all possible gadgets.
#                                      The script will be run as 'exploit-script $offset'.
#         --info BuildID               Show version information given BuildID.
#         --base BASE_ADDRESS          The base address of libc.
#                                      Default: 0
#         --version                    Current gem version.

$ one_gadget spec/data/libc-2.31-9fdb74e7b217d06c93172a8243f8547f947ee6d1.so
# 0xe3b31 execve("/bin/sh", r15, rdx)
# constraints:
#   [r15] == NULL || r15 == NULL || r15 is a valid argv
#   [rdx] == NULL || rdx == NULL || rdx is a valid envp
#
# 0xe3b34 execve("/bin/sh", rsi, rdx)
# constraints:
#   [rsi] == NULL || rsi == NULL || rsi is a valid argv
#   [rdx] == NULL || rdx == NULL || rdx is a valid envp
#
# 0xe3c20 execve("/bin/sh", r15, r12)
# constraints:
#   [r15] == NULL || r15 == NULL || r15 is a valid argv
#   [r12] == NULL || r12 == NULL || r12 is a valid envp
#
# 0x107cea posix_spawn(rsp+0x64, "/bin/sh", [rsp+0x38], 0, rsp+0x70, environ)
# constraints:
#   [rsp+0x70] == NULL || {[rsp+0x70], [rsp+0x78], [rsp+0x80], [rsp+0x88], ...} is a valid argv
#   [rsp+0x38] == NULL || (s32)[[rsp+0x38]+0x4] <= 0x0

Given BuildID

$ one_gadget -b b417c0ba7cc5cf06d1d1bed6652cedb9253c60d0
# 0x4f322 execve("/bin/sh", rsp+0x40, environ)
# constraints:
#   [rsp+0x40] == NULL || {[rsp+0x40], [rsp+0x48], [rsp+0x50], [rsp+0x58], ...} is a valid argv
#
# 0xe56d8 execve("/bin/sh", r14, r12)
# constraints:
#   [r14] == NULL || r14 == NULL || r14 is a valid argv
#   [r12] == NULL || r12 == NULL || r12 is a valid envp
#
# 0xe5863 execve("/bin/sh", r10, rdx)
# constraints:
#   [r10] == NULL || r10 == NULL || r10 is a valid argv
#   [rdx] == NULL || rdx == NULL || rdx is a valid envp
#
# 0x10a38c execve("/bin/sh", rsp+0x70, environ)
# constraints:
#   [rsp+0x70] == NULL || {[rsp+0x70], [rsp+0x78], [rsp+0x80], [rsp+0x88], ...} is a valid argv

The gem carries the gadgets of the libcs most likely to be asked for -- the glibc of every Ubuntu LTS still in standard support, and every libc the tests cover. Any other BuildID is fetched from this repository, which keeps them all.

Gadgets Near Functions

Why

Consider this situation while exploiting:

You can write to the GOT (Global Offset Table).
The base address of libc is unknown.

Here you can overwrite the low two bytes of a GOT entry with the low two bytes of a one-gadget. If the function sits close enough to the gadget, only one nibble is left to guess -- at least a 1 in 16 chance of success.

Usage

Sorts the gadgets by how far they are from the functions you name.

$ one_gadget spec/data/libc-2.31-9fdb74e7b217d06c93172a8243f8547f947ee6d1.so --near exit,mkdir
# [OneGadget] Gadgets near exit(0x46a70):
# 0xe3b31 execve("/bin/sh", r15, rdx)
# constraints:
#   [r15] == NULL || r15 == NULL || r15 is a valid argv
#   [rdx] == NULL || rdx == NULL || rdx is a valid envp
#
# 0xe3b34 execve("/bin/sh", rsi, rdx)
# constraints:
#   [rsi] == NULL || rsi == NULL || rsi is a valid argv
#   [rdx] == NULL || rdx == NULL || rdx is a valid envp
#
# 0xe3c20 execve("/bin/sh", r15, r12)
# constraints:
#   [r15] == NULL || r15 == NULL || r15 is a valid argv
#   [r12] == NULL || r12 == NULL || r12 is a valid envp
#
# 0x107cea posix_spawn(rsp+0x64, "/bin/sh", [rsp+0x38], 0, rsp+0x70, environ)
# constraints:
#   [rsp+0x70] == NULL || {[rsp+0x70], [rsp+0x78], [rsp+0x80], [rsp+0x88], ...} is a valid argv
#   [rsp+0x38] == NULL || (s32)[[rsp+0x38]+0x4] <= 0x0
#
# [OneGadget] Gadgets near mkdir(0x10dc80):
# 0x107cea posix_spawn(rsp+0x64, "/bin/sh", [rsp+0x38], 0, rsp+0x70, environ)
# constraints:
#   [rsp+0x70] == NULL || {[rsp+0x70], [rsp+0x78], [rsp+0x80], [rsp+0x88], ...} is a valid argv
#   [rsp+0x38] == NULL || (s32)[[rsp+0x38]+0x4] <= 0x0
#
# 0xe3c20 execve("/bin/sh", r15, r12)
# constraints:
#   [r15] == NULL || r15 == NULL || r15 is a valid argv
#   [r12] == NULL || r12 == NULL || r12 is a valid envp
#
# 0xe3b34 execve("/bin/sh", rsi, rdx)
# constraints:
#   [rsi] == NULL || rsi == NULL || rsi is a valid argv
#   [rdx] == NULL || rdx == NULL || rdx is a valid envp
#
# 0xe3b31 execve("/bin/sh", r15, rdx)
# constraints:
#   [r15] == NULL || r15 == NULL || r15 is a valid argv
#   [rdx] == NULL || rdx == NULL || rdx is a valid envp
#

Regular expressions work too.

$ one_gadget spec/data/libc-2.31-9fdb74e7b217d06c93172a8243f8547f947ee6d1.so --near 'write.*' --raw
# [OneGadget] Gadgets near writev(0x1144a0):
# 1080554 932896 932660 932657
#
# [OneGadget] Gadgets near write(0x10e090):
# 1080554 932896 932660 932657
#

Pass an ELF file instead, and OneGadget uses every function in its GOT.

$ one_gadget spec/data/libc-2.31-9fdb74e7b217d06c93172a8243f8547f947ee6d1.so --near spec/data/test_near_file.elf --raw
# [OneGadget] Gadgets near exit(0x46a70):
# 932657 932660 932896 1080554
#
# [OneGadget] Gadgets near puts(0x84450):
# 932657 932660 932896 1080554
#
# [OneGadget] Gadgets near printf(0x61cc0):
# 932657 932660 932896 1080554
#
# [OneGadget] Gadgets near strlen(0x9f660):
# 932657 932660 932896 1080554
#
# [OneGadget] Gadgets near __cxa_finalize(0x46f40):
# 932657 932660 932896 1080554
#
# [OneGadget] Gadgets near __libc_start_main(0x23fc0):
# 932657 932660 932896 1080554
#

Show More Gadgets

A libc holds far more gadgets than fit on a screen, and most of them ask more of you than a handful of others do. --level chooses how much of what was found to show.

Level	Shows	Use it when
`0` (default)	the few gadgets whose constraints are easiest to arrange	always, first
`1`	every gadget, one line per address, minus any that asks for everything an easier one asks for and more	none of the level-0 gadgets fit what you control
`2`	everything found, including an address listed once per set of constraints it can be reached under	you want to see every way in, including the awkward ones

Level 1 drops a gadget when an easier one already asks for everything it does; level 0 then scores what is left by how hard its constraints look and keeps only the best few; level 2 drops nothing. On the glibc 2.31 below that is 4, 32 and 97 entries -- the last over 96 addresses, since one of them is reachable two ways with different requirements.

Level 2 reads the libc you point at directly, where the levels below it answer from the pre-built gadget database when it knows the file's BuildID -- the same gadgets either way. Pass -f to search the file at every level.

$ one_gadget spec/data/libc-2.31-9fdb74e7b217d06c93172a8243f8547f947ee6d1.so --level 1
# 0x51df8 posix_spawn(rsp+0xc, "/bin/sh", 0, rsp+0x210, rsp+0x50, environ)
# constraints:
#   readable: r12
#   readable: r13
#   rsp & 0xf == 0x0
#   {"sh", "-c", rbx, NULL} is a valid argv
#   rsp+0x210 == NULL || (u16)[rsp+0x210] == 0x0
#
# 0x51e2b posix_spawn(rsp+0xc, "/bin/sh", 0, rbp, rsp+0x50, environ)
# constraints:
#   rsp & 0xf == 0x0
#   {"sh", "-c", rbx, NULL} is a valid argv
#   rbp == NULL || (u16)[rbp] == 0x0
#
# 0x51e32 posix_spawn(rsp+0xc, "/bin/sh", 0, rbp, rsp+0x50, environ)
# constraints:
#   rsp & 0xf == 0x0
#   rax == NULL || {"sh", rax, rbx, NULL} is a valid argv
#   rbp == NULL || (u16)[rbp] == 0x0
#
# 0x51e39 posix_spawn(rsp+0xc, "/bin/sh", 0, rbp, rsp+0x50, environ)
# constraints:
#   rsp & 0xf == 0x0
#   rcx == NULL || {rcx, rax, rbx, NULL} is a valid argv
#   rbp == NULL || (u16)[rbp] == 0x0
#
# 0x51e40 posix_spawn(rsp+0xc, "/bin/sh", rdx, rbp, rsp+0x50, environ)
# constraints:
#   rsp & 0xf == 0x0
#   rcx == NULL || {rcx, (u64)xmm1, rbx, NULL} is a valid argv
#   rdx == NULL || (s32)[rdx+0x4] <= 0x0
#   rbp == NULL || (u16)[rbp] == 0x0
#
# 0x51e45 posix_spawn(rsp+0xc, "/bin/sh", rdx, rbp, rsp+0x50, environ)
# constraints:
#   rsp & 0xf == 0x0
#   (u64)xmm0 == NULL || {(u64)xmm0, (u64)xmm1, rbx, NULL} is a valid argv
#   rdx == NULL || (s32)[rdx+0x4] <= 0x0
#   rbp == NULL || (u16)[rbp] == 0x0
#
# 0x51e55 posix_spawn(rdi, "/bin/sh", rdx, rbp, rsp+0x50, [rax])
# constraints:
#   readable: rax
#   rsp & 0xf == 0x0
#   (u64)xmm0 == NULL || {(u64)xmm0, (u64)(xmm0 >> 64), rbx, NULL} is a valid argv
#   [[rax]] == NULL || [rax] == NULL || [rax] is a valid envp
#   rdi == NULL || writable: rdi
#   rdx == NULL || (s32)[rdx+0x4] <= 0x0
#   rbp == NULL || (u16)[rbp] == 0x0
#
# 0x51e5a posix_spawn(rdi, "/bin/sh", rdx, rbp, r8, [rax])
# constraints:
#   readable: rax
#   rsp & 0xf == 0x0
#   [r8] == NULL || r8 is a valid argv
#   [[rax]] == NULL || [rax] == NULL || [rax] is a valid envp
#   rdi == NULL || writable: rdi
#   rdx == NULL || (s32)[rdx+0x4] <= 0x0
#   rbp == NULL || (u16)[rbp] == 0x0
#
# 0x84165 posix_spawn(rbx+0xe0, "/bin/sh", r12, 0, rsp+0x60, environ)
# constraints:
#   rsp & 0xf == 0x0
#   {"sh", "-c", rbp, NULL} is a valid argv
#   rbx+0xe0 == NULL || writable: rbx+0xe0
#   r12 == NULL || (s32)[r12+0x4] <= 0x0
#
# 0x8416c posix_spawn(rbx+0xe0, "/bin/sh", r12, 0, rsp+0x60, environ)
# constraints:
#   rsp & 0xf == 0x0
#   rax == NULL || {"sh", rax, rbp, NULL} is a valid argv
#   rbx+0xe0 == NULL || writable: rbx+0xe0
#   r12 == NULL || (s32)[r12+0x4] <= 0x0
#
# 0x84173 posix_spawn(rbx+0xe0, "/bin/sh", r12, 0, rsp+0x60, environ)
# constraints:
#   rsp & 0xf == 0x0
#   rcx == NULL || {rcx, rax, rbp, NULL} is a valid argv
#   rbx+0xe0 == NULL || writable: rbx+0xe0
#   r12 == NULL || (s32)[r12+0x4] <= 0x0
#
# 0x84176 posix_spawn(rbx+0xe0, "/bin/sh", rdx, 0, rsp+0x60, environ)
# constraints:
#   rsp & 0xf == 0x0
#   rcx == NULL || {rcx, rax, rbp, NULL} is a valid argv
#   rbx+0xe0 == NULL || writable: rbx+0xe0
#   rdx == NULL || (s32)[rdx+0x4] <= 0x0
#
# 0x8417b posix_spawn(rbx+0xe0, "/bin/sh", rdx, 0, rsp+0x60, environ)
# constraints:
#   rsp & 0xf == 0x0
#   rcx == NULL || {rcx, rax, [rsp+0x70], NULL} is a valid argv
#   rbx+0xe0 == NULL || writable: rbx+0xe0
#   rdx == NULL || (s32)[rdx+0x4] <= 0x0
#
# 0x84180 posix_spawn(rbx+0xe0, "/bin/sh", rdx, 0, rsp+0x60, environ)
# constraints:
#   rsp & 0xf == 0x0
#   rcx == NULL || {rcx, (u64)xmm1, [rsp+0x70], NULL} is a valid argv
#   rbx+0xe0 == NULL || writable: rbx+0xe0
#   rdx == NULL || (s32)[rdx+0x4] <= 0x0
#
# 0x8418c posix_spawn(rbx+0xe0, "/bin/sh", rdx, 0, rsp+0x60, [rax])
# constraints:
#   readable: rax
#   rsp & 0xf == 0x0
#   (u64)xmm0 == NULL || {(u64)xmm0, (u64)xmm1, [rsp+0x70], NULL} is a valid argv
#   [[rax]] == NULL || [rax] == NULL || [rax] is a valid envp
#   rbx+0xe0 == NULL || writable: rbx+0xe0
#   rdx == NULL || (s32)[rdx+0x4] <= 0x0
#
# 0x84192 posix_spawn(rbx+0xe0, "/bin/sh", rdx, rcx, rsp+0x60, [rax])
# constraints:
#   readable: rax
#   rsp & 0xf == 0x0
#   (u64)xmm0 == NULL || {(u64)xmm0, (u64)(xmm0 >> 64), [rsp+0x70], NULL} is a valid argv
#   [[rax]] == NULL || [rax] == NULL || [rax] is a valid envp
#   rbx+0xe0 == NULL || writable: rbx+0xe0
#   rdx == NULL || (s32)[rdx+0x4] <= 0x0
#   rcx == NULL || (u16)[rcx] == 0x0
#
# 0x84199 posix_spawn(rdi, "/bin/sh", rdx, rcx, rsp+0x60, [rax])
# constraints:
#   readable: rax
#   rsp & 0xf == 0x0
#   (u64)xmm0 == NULL || {(u64)xmm0, (u64)(xmm0 >> 64), [rsp+0x70], NULL} is a valid argv
#   [[rax]] == NULL || [rax] == NULL || [rax] is a valid envp
#   rdi == NULL || writable: rdi
#   rdx == NULL || (s32)[rdx+0x4] <= 0x0
#   rcx == NULL || (u16)[rcx] == 0x0
#
# 0x841a0 posix_spawn(rdi, "/bin/sh", rdx, rcx, r8, [rax])
# constraints:
#   readable: rax
#   rsp & 0xf == 0x0
#   [r8] == NULL || r8 is a valid argv
#   [[rax]] == NULL || [rax] == NULL || [rax] is a valid envp
#   rdi == NULL || writable: rdi
#   rdx == NULL || (s32)[rdx+0x4] <= 0x0
#   rcx == NULL || (u16)[rcx] == 0x0
#
# 0xe3b31 execve("/bin/sh", r15, rdx)
# constraints:
#   [r15] == NULL || r15 == NULL || r15 is a valid argv
#   [rdx] == NULL || rdx == NULL || rdx is a valid envp
#
# 0xe3b34 execve("/bin/sh", rsi, rdx)
# constraints:
#   [rsi] == NULL || rsi == NULL || rsi is a valid argv
#   [rdx] == NULL || rdx == NULL || rdx is a valid envp
#
# 0xe3c20 execve("/bin/sh", r15, r12)
# constraints:
#   [r15] == NULL || r15 == NULL || r15 is a valid argv
#   [r12] == NULL || r12 == NULL || r12 is a valid envp
#
# 0xe3d26 execve("/bin/sh", r10, rdx)
# constraints:
#   address rbp-0x78 is writable
#   [r10] == NULL || r10 == NULL || r10 is a valid argv
#   [rdx] == NULL || rdx == NULL || rdx is a valid envp
#
# 0xe3d88 execve("/bin/sh", [rbp-0x78], r12)
# constraints:
#   address rbp-0x78 is writable
#   [[rbp-0x78]] == NULL || [rbp-0x78] == NULL || [rbp-0x78] is a valid argv
#   [r12] == NULL || r12 == NULL || r12 is a valid envp
#
# 0xe3d90 execve("/bin/sh", r10, r12)
# constraints:
#   address rbp-0x78 is writable
#   [r10] == NULL || r10 == NULL || r10 is a valid argv
#   [r12] == NULL || r12 == NULL || r12 is a valid envp
#
# 0xe3d92 execve("/bin/sh", rbp-0x50, r12)
# constraints:
#   address rbp-0x50 is writable
#   r13 == NULL || {"/bin/sh", r13, NULL} is a valid argv
#   [r12] == NULL || r12 == NULL || r12 is a valid envp
#
# 0xe3d99 execve("/bin/sh", rbp-0x50, r12)
# constraints:
#   address rbp-0x50 is writable
#   rax == NULL || {rax, r13, NULL} is a valid argv
#   [r12] == NULL || r12 == NULL || r12 is a valid envp
#
# 0xe3da0 execve("/bin/sh", rbp-0x50, r12)
# constraints:
#   address rbp-0x50 is writable
#   rax == NULL || {rax, [rbp-0x48], NULL} is a valid argv
#   [r12] == NULL || r12 == NULL || r12 is a valid envp
#
# 0xe3dd7 execve("/bin/sh", rbp-0x50, r12)
# constraints:
#   address rbp-0x78 is writable
#   [rbp-0x68] == NULL || {"/bin/sh", [rbp-0x68], NULL} is a valid argv
#   [r12] == NULL || r12 == NULL || r12 is a valid envp
#
# 0xe3de1 execve("/bin/sh", rbp-0x50, r12)
# constraints:
#   address rbp-0x78 is writable
#   rax == NULL || {rax, [rbp-0x68], NULL} is a valid argv
#   [r12] == NULL || r12 == NULL || r12 is a valid envp
#
# 0x1075e7 posix_spawn(rsp+0x64, "/bin/sh", rdx, 0, rsp+0x70, r9)
# constraints:
#   [rsp+0x70] == NULL || {[rsp+0x70], [rsp+0x78], [rsp+0x80], [rsp+0x88], ...} is a valid argv
#   [r9] == NULL || r9 == NULL || r9 is a valid envp
#   rdx == NULL || (s32)[rdx+0x4] <= 0x0
#
# 0x1075f1 posix_spawn(rdi, "/bin/sh", rdx, 0, r8, r9)
# constraints:
#   [r8] == NULL || r8 is a valid argv
#   [r9] == NULL || r9 == NULL || r9 is a valid envp
#   rdi == NULL || writable: rdi
#   rdx == NULL || (s32)[rdx+0x4] <= 0x0
#
# 0x107cea posix_spawn(rsp+0x64, "/bin/sh", [rsp+0x38], 0, rsp+0x70, environ)
# constraints:
#   [rsp+0x70] == NULL || {[rsp+0x70], [rsp+0x78], [rsp+0x80], [rsp+0x88], ...} is a valid argv
#   [rsp+0x38] == NULL || (s32)[[rsp+0x38]+0x4] <= 0x0

Other Architectures

i386

$ one_gadget spec/data/libc-2.27-63b3d43ad45e1b0f601848c65b067f9e9b40528b.so
# 0x3cbf7 execve("/bin/sh", esp+0x40, environ)
# constraints:
#   esi is the GOT address of libc
#   [esp+0x40] == NULL || {[esp+0x40], [esp+0x44], [esp+0x48], [esp+0x4c], ...} is a valid argv
#
# 0x6729f execl("/bin/sh", eax)
# constraints:
#   esi is the GOT address of libc
#   eax == NULL
#
# 0x672a0 execl("/bin/sh", [esp])
# constraints:
#   esi is the GOT address of libc
#   [esp] == NULL
#
# 0x13573e execl("/bin/sh", eax)
# constraints:
#   ebx is the GOT address of libc
#   eax == NULL
#
# 0x13573f execl("/bin/sh", [esp])
# constraints:
#   ebx is the GOT address of libc
#   [esp] == NULL

AArch64

$ one_gadget spec/data/aarch64-libc-2.27.so
# 0x3f160 execve("/bin/sh", sp+0x70, environ)
# constraints:
#   address x20+0x338 is writable
#   x22 == NULL
#   x3 == NULL || {x3, "-c", x23, NULL} is a valid argv
#
# 0x63e90 execl("/bin/sh", x1)
# constraints:
#   x1 == NULL
#
# 0xa321c execve("/bin/sh", sp+0x40, x2)
# constraints:
#   [x1] == 0x0
#   x0 == NULL || {"/bin/sh", x0, NULL} is a valid argv
#   [x2] == NULL || x2 == NULL || x2 is a valid envp
#
# 0xa32e8 execve("/bin/sh", x29+0x40, x23)
# constraints:
#   address x29+0x40 is writable
#   x0 == NULL || {"/bin/sh", x0, NULL} is a valid argv
#   [x23] == NULL || x23 == NULL || x23 is a valid envp

ARM32

$ one_gadget spec/data/arm-libc-2.39.so
# 0x38f6c posix_spawn(r0, "/bin/sh", r2, r8, [sp], r3)
# constraints:
#   [[sp]] == NULL || [sp] is a valid argv
#   [r3] == NULL || r3 == NULL || r3 is a valid envp
#   r0 == NULL || writable: r0
#   r2 == NULL || (s32)[r2+0x4] <= 0x0
#   r8 == NULL || (u16)[r8] == 0x0
#
# 0x88a7c execve("/bin/sh", r4, r5)
# constraints:
#   [r4] == NULL || r4 == NULL || r4 is a valid argv
#   [r5] == NULL || r5 == NULL || r5 is a valid envp
#
# 0x9f2be posix_spawn([sp+0x34], "/bin/sh", [sp+0x2c], 0, [sp+0x3c], r3)
# constraints:
#   [[sp+0x3c]] == NULL || [sp+0x3c] is a valid argv
#   [r3] == NULL || r3 == NULL || r3 is a valid envp
#   [sp+0x34] == NULL || writable: [sp+0x34]
#   [sp+0x2c] == NULL || (s32)[[sp+0x2c]+0x4] <= 0x0

RISC-V (RV64)

$ one_gadget spec/data/riscv64-libc-2.39.so
# 0x9f73a execve("/bin/sh", s1, a2)
# constraints:
#   [s1] == NULL || s1 == NULL || s1 is a valid argv
#   [a2] == NULL || a2 == NULL || a2 is a valid envp
#
# 0x9f73c execve("/bin/sh", a1, a2)
# constraints:
#   [a1] == NULL || a1 == NULL || a1 is a valid argv
#   [a2] == NULL || a2 == NULL || a2 is a valid envp
#
# 0x9f778 execve("/bin/sh", s1, s2)
# constraints:
#   [s1] == NULL || s1 == NULL || s1 is a valid argv
#   [s2] == NULL || s2 == NULL || s2 is a valid envp
#
# 0xb5adc posix_spawn(sp+0x44, "/bin/sh", [sp+0x30], 0, sp+0x50, environ)
# constraints:
#   [sp+0x50] == NULL || {[sp+0x50], [sp+0x58], [sp+0x60], [sp+0x68], ...} is a valid argv
#   [sp+0x30] == NULL || (s32)[[sp+0x30]+0x4] <= 0x0

MIPS

Big-endian is as common as little-endian here, and both are read. This one is OpenWrt's musl for ath79, which ships with no section headers -- the shape a router's libc usually arrives in.

$ one_gadget spec/data/mips-musl-1.2.4.so
# 0x668e8 posix_spawn(sp+0x20, "/bin/sh", 0, s0, sp+0x28, environ)
# constraints:
#   gp is the GOT address of libc
#   {"sh", "-c", s1, NULL} is a valid argv
#   s0 == NULL || (u16)[s0] == 0x0
#
# 0x668ec posix_spawn(sp+0x20, "/bin/sh", 0, s0, sp+0x28, environ)
# constraints:
#   gp is the GOT address of libc
#   {"sh", "-c", [sp+0x30], NULL} is a valid argv
#   s0 == NULL || (u16)[s0] == 0x0
#
# 0x668f0 posix_spawn(sp+0x20, "/bin/sh", 0, a3, sp+0x28, environ)
# constraints:
#   gp is the GOT address of libc
#   {"sh", "-c", [sp+0x30], NULL} is a valid argv
#   a3 == NULL || (u16)[a3] == 0x0
#
# 0x668f4 posix_spawn(sp+0x20, "/bin/sh", 0, a3, sp+0x28, environ)
# constraints:
#   gp is the GOT address of libc
#   v0-0x7c70 == NULL || {v0-0x7c70, "-c", [sp+0x30], NULL} is a valid argv
#   a3 == NULL || (u16)[a3] == 0x0
#
# 0x77b44 posix_spawn(sp+0x20, "/bin/sh", s2, 0, sp+0x2c, environ)
# constraints:
#   gp is the GOT address of libc
#   {"sh", "-c", s7, NULL} is a valid argv
#   s2 == NULL || (s32)[s2+0x4] <= 0x0
#
# 0x77b48 posix_spawn(sp+0x20, "/bin/sh", s2, 0, sp+0x2c, environ)
# constraints:
#   gp is the GOT address of libc
#   v0-0x7c70 == NULL || {v0-0x7c70, "-c", s7, NULL} is a valid argv
#   s2 == NULL || (s32)[s2+0x4] <= 0x0

Combine with Script

Pass your exploit script to one_gadget and it runs the script once per gadget, so you don't have to work through them by hand.

$ one_gadget spec/data/libc-2.31-9fdb74e7b217d06c93172a8243f8547f947ee6d1.so -s 'echo "offset ->"'

In Ruby Scripts

require 'one_gadget'
OneGadget.gadgets(file: 'spec/data/libc-2.31-9fdb74e7b217d06c93172a8243f8547f947ee6d1.so')
#=> [932657, 932660, 932896, 1080554]

# or, more briefly
one_gadget('spec/data/libc-2.31-9fdb74e7b217d06c93172a8243f8547f947ee6d1.so', level: 1)
#=> [335352, 335403, 335410, 335417, 335424, 335429, 335445, 335450, 541029, 541036, 541043, 541046, 541051, 541056, 541068, 541074, 541081, 541088, 932657, 932660, 932896, 933158, 933256, 933264, 933266, 933273, 933280, 933335, 933345, 1078759, 1078769, 1080554]

# from build id
one_gadget('b417c0ba7cc5cf06d1d1bed6652cedb9253c60d0')
#=> [324386, 939736, 940131, 1090444]

To Python Lovers

import subprocess
def one_gadget(filename):
  return [int(i) for i in subprocess.check_output(['one_gadget', '--raw', filename]).decode().split(' ')]

one_gadget('spec/data/libc-2.31-9fdb74e7b217d06c93172a8243f8547f947ee6d1.so')
#=> [932657, 932660, 932896, 1080554]

Make OneGadget Better

Any suggestion or feature request is welcome, and pull requests even more so!

Please let me know if you come across a libc OneGadget finds nothing in. And if you like this work, I'd be happy to be starred 😬