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In a 64 bit ELF binary I found it mainly uses fs register to get some values. How can I know which value it wants to access? I'm familiar with that NT kernel uses fs and gs register for TEB structure in 32 bit and 64 bit OS respectively.
Here are two examples:
1.
mov rax, fs:28h
mov [rsp+88], rax
2.
sub_a proc near
mov rax, fs:0
add rax, 44h
retn
sub_a endp
The gs/fs segment can be used for thread local storage similar to what you have encountered in Windows. Variable specific to a thread such as errno, stack canary etc are usually stored here in Linux.
According to this, your first example is to save canary to the stack from fs:0x28. You can see some hacks here and read more here
Canary check from an example binary
$ tail x.c
#include <stdio.h>
int main(int argc, char **argv)
{
char s[32];
scanf("%s", s);
return 0;
}
$ gcc -no-pie -fno-pic x.c -o z64
$ gcc -m32 -no-pie -fno-pic x.c -o z32
$ r2 -AA z32 -qc "pdf @ sym.main"
;-- main:
┌ (fcn) sym.main 86
....
│ 0x080484bf 65a114000000 mov eax, dword gs:[0x14] ; [0x14:4]=-1 ; 20
│ 0x080484c5 8945f4 mov dword [local_ch], eax
│ 0x080484c8 31c0 xor eax, eax
....
│ 0x080484e3 8b55f4 mov edx, dword [local_ch]
│ 0x080484e6 653315140000. xor edx, dword gs:[0x14]
│ ┌─< 0x080484ed 7405 je 0x80484f4
│ │ 0x080484ef e85cfeffff call sym.imp.__stack_chk_fail ; void __stack_chk_fail(void)
....
$ r2 -AA z64 -qc "pdf @ sym.main"
;-- main:
┌ (fcn) sym.main 79
....
│ 0x00400586 64488b042528. mov rax, qword fs:[0x28] ; [0x28:8]=-1 ; '(' ; 40
│ 0x0040058f 488945f8 mov qword [local_8h], rax
│ 0x00400593 31c0 xor eax, eax
....
│ 0x004005b0 488b55f8 mov rdx, qword [local_8h]
│ 0x004005b4 644833142528. xor rdx, qword fs:[0x28]
│ ┌─< 0x004005bd 7405 je 0x4005c4
│ │ 0x004005bf e8acfeffff call sym.imp.__stack_chk_fail ; void __stack_chk_fail(void)
....
Also for your second request, yes that is pthread_t at offset 0 in fs/gs.
$ tail -n 30 x.c
#ifdef __x86_64__
#define val_t uint64_t
#define INSN_READ "movq %%fs:0, %0;"
#define FMT "Found val: %#lx\n"
#elif __i386__
#define val_t uint32_t
#define INSN_READ "movl %%gs:0, %0;"
#define FMT "Found val: %#x\n"
#endif
val_t read_val()
{
val_t val = 0;
__asm__(INSN_READ
: "=r"(val)
:
:);
return val;
}
int main(int argc, char **argv)
{
printf(FMT, read_val());
printf(FMT, (val_t)pthread_self());
return 0;
}
$ gcc -no-pie -fno-pic x.c -o zm64
$ gcc -m32 -no-pie -fno-pic x.c -o zm32
$ ./zm32
Found val: 0xf7f800c0
Found val: 0xf7f800c0
$ ./zm64
Found val: 0x7fd50119f4c0
Found val: 0x7fd50119f4c0
I tried to look for your second snippet in my libc but couldn't find it. SO can't answer for sure what it contains at 0x44. While debugging I only got 0s at that offset.
$ r2 /lib/x86_64-linux-gnu/libc.so.6
-- There is only one binary, and we are all just reversing pieces of it.
[0x00021cb0]> /x `!rasm2 -a x86.as -b 64 "mov rax, fs:[0]"`
Searching 9 bytes in [0x0-0x1e6aa0]
hits: 7
Searching 9 bytes in [0x3e7620-0x3f0ae0]
hits: 0
0x000e18c9 hit0_0 64488b042500000000
0x000e1d8a hit0_1 64488b042500000000
0x00105cff hit0_2 64488b042500000000
0x00105e50 hit0_3 64488b042500000000
0x00105f21 hit0_4 64488b042500000000
0x00106865 hit0_5 64488b042500000000
0x0014a18a hit0_6 64488b042500000000
[0x00021cb0]> pd10 @@ hit0_*
;-- hit0_0:
0x000e18c9 64488b042500. mov rax, qword fs:[0]
0x000e18d2 31db xor ebx, ebx
0x000e18d4 48c744241800. mov qword [rsp + 0x18], 0
0x000e18dd 4c8b25849530. mov r12, qword [0x003eae68] ; [0x3eae68:8]=0
0x000e18e4 48890424 mov qword [rsp], rax
0x000e18e8 488d442450 lea rax, [rsp + 0x50] ; "@" ; 'P'
0x000e18ed 4889442430 mov qword [rsp + 0x30], rax
0x000e18f2 488d442460 lea rax, [rsp + 0x60] ; '`'
0x000e18f7 4889442420 mov qword [rsp + 0x20], rax
0x000e18fc 488d442458 lea rax, [rsp + 0x58] ; "@" ; 'X'
;-- hit0_1:
0x000e1d8a 64488b042500. mov rax, qword fs:[0]
0x000e1d93 31db xor ebx, ebx
0x000e1d95 48c744242800. mov qword [rsp + 0x28], 0
0x000e1d9e 4c8b25c39030. mov r12, qword [0x003eae68] ; [0x3eae68:8]=0
0x000e1da5 4889442410 mov qword [rsp + 0x10], rax
0x000e1daa 488d442460 lea rax, [rsp + 0x60] ; '`'
0x000e1daf 4889442440 mov qword [rsp + 0x40], rax
0x000e1db4 488d442470 lea rax, [rsp + 0x70] ; 'p'
0x000e1db9 4889442430 mov qword [rsp + 0x30], rax
0x000e1dbe 488d442468 lea rax, [rsp + 0x68] ; 'h'
;-- hit0_2:
0x00105cff 64488b042500. mov rax, qword fs:[0]
0x00105d08 488b1d41512e. mov rbx, qword [0x003eae50] ; [0x3eae50:8]=0
0x00105d0f 4c8b2d52512e. mov r13, qword [0x003eae68] ; [0x3eae68:8]=0
0x00105d16 4c8d3c18 lea r15, [rax + rbx]
0x00105d1a 48898560feff. mov qword [rbp - 0x1a0], rax
0x00105d21 4c89e7 mov rdi, r12
0x00105d24 e807060600 call sym._dl_mcount_wrapper_check
0x00105d29 488b8560feff. mov rax, qword [rbp - 0x1a0]
0x00105d30 4883ec08 sub rsp, 8
0x00105d34 498b4e08 mov rcx, qword [r14 + 8] ; sym.__resp ; [0x8:8]=0
;-- hit0_3:
0x00105e50 64488b042500. mov rax, qword fs:[0]
0x00105e59 4c8b2d08502e. mov r13, qword [0x003eae68] ; [0x3eae68:8]=0
0x00105e60 4c8dbd98feff. lea r15, [rbp - 0x168]
0x00105e67 488b1de24f2e. mov rbx, qword [0x003eae50] ; [0x3eae50:8]=0
0x00105e6e 48c78598feff. mov qword [rbp - 0x168], 0
0x00105e79 48898560feff. mov qword [rbp - 0x1a0], rax
0x00105e80 4c01e8 add rax, r13 ; 'o'
0x00105e83 48898508feff. mov qword [rbp - 0x1f8], rax
0x00105e8a 660f1f440000 nop word [rax + rax]
0x00105e90 4c89e7 mov rdi, r12
;-- hit0_4:
0x00105f21 64488b042500. mov rax, qword fs:[0]
0x00105f2a 4c8b2d374f2e. mov r13, qword [0x003eae68] ; [0x3eae68:8]=0
0x00105f31 4c8dbd98feff. lea r15, [rbp - 0x168]
0x00105f38 488b1d114f2e. mov rbx, qword [0x003eae50] ; [0x3eae50:8]=0
0x00105f3f 48c78598feff. mov qword [rbp - 0x168], 0
0x00105f4a 48898560feff. mov qword [rbp - 0x1a0], rax
0x00105f51 4c01e8 add rax, r13 ; 'o'
0x00105f54 48898508feff. mov qword [rbp - 0x1f8], rax
0x00105f5b 0f1f440000 nop dword [rax + rax]
0x00105f60 4c89e7 mov rdi, r12
;-- hit0_5:
0x00106865 64488b042500. mov rax, qword fs:[0]
0x0010686e 488db5c0feff. lea rsi, [rbp - 0x140]
0x00106875 4c8b0dd4452e. mov r9, qword [0x003eae50] ; [0x3eae50:8]=0
0x0010687c 488b8d20feff. mov rcx, qword [rbp - 0x1e0]
0x00106883 ba00010000 mov edx, 0x100
0x00106888 4885ff test rdi, rdi
0x0010688b 490f44fd cmove rdi, r13
0x0010688f 4901c1 add r9, rax ; '#'
0x00106892 480305cf452e. add rax, qword [0x003eae68]
0x00106899 4989c0 mov r8, rax
;-- hit0_6:
0x0014a18a 64488b042500. mov rax, qword fs:[0]
0x0014a193 4c89fb mov rbx, r15
0x0014a196 4889442440 mov qword [rsp + 0x40], rax
0x0014a19b 488d442460 lea rax, [rsp + 0x60] ; '`'
0x0014a1a0 4889442408 mov qword [rsp + 8], rax
0x0014a1a5 488b4500 mov rax, qword [rbp]
0x0014a1a9 488b7c2458 mov rdi, qword [rsp + 0x58] ; [0x58:8]=64 ; 'X'
0x0014a1ae 4889442410 mov qword [rsp + 0x10], rax
0x0014a1b3 488b03 mov rax, qword [rbx]
0x0014a1b6 4889442418 mov qword [rsp + 0x18], rax
[0x00021cb0]>
rax + 0x44.
Nov 28 '18 at 7:15
sys/types.h (e.g. in /usr/include/x86_64-linux-gnu/sys/types.h) ... which in turn includes pthreadtypes.h (e.g. /usr/include/x86_64-linux-gnu/bits/pthreadtypes.h) which has typedef unsigned long int pthread_t;. But mind that this is system/architecture-specific.
pthread_t which I gave you above ...
rax+0x40 is the offset of errno (4 bytes takes rax+0x44). There may be some relation between pthread and errno.
Nov 28 '18 at 18:10