10 Jun
2016
10 Jun
'16
4:44 p.m.
The 'BLT' instruction checks for *signed* values. So if a3, length
parameter of memset, is negative, then value added to the PC will be
large.
memset(buf, 0xaa, 0xffff0000) triggers the bug.
GDB session without the patch:
"""
$ gdb ./main-buggy-memset.elf -q
Reading symbols from ./main-buggy-memset.elf...done.
(gdb) x/i memset
0x8770 <memset>: mov r3, r0
(gdb) r
Starting program: /root/memset/main-buggy-memset.elf
Program received signal SIGSEGV, Segmentation fault.
0x00048808 in ?? ()
"""
The $pc is outside of the memset function because:
"""
(gdb) x/i $pc
=> 0x87e4 <memset+116>: add pc, pc, r2, lsl #2
(gdb) info reg $r2
r2 0x10007 65543
"""
GDB session with the bug fixed (patch applied):
"""
$ gdb ./main-fixed-memset.elf -q
Reading symbols from ./main-fixed-memset.elf...done.
(gdb) x/i memset
0x8770 <memset>: mov r3, r0
(gdb) r
Starting program: /root/memset/main-fixed-memset.elf
Program received signal SIGSEGV, Segmentation fault.
memset () at libc/string/arm/memset.S:92
92 libc/string/arm/memset.S: No such file or directory.
(gdb) x/i $pc
=> 0x87b0 <memset+64>: stmia r3!, {r1, r12}
(gdb) info reg $r3
r3 0x15000 86016
(gdb) info proc mappings
process 5822
Mapped address spaces:
Start Addr End Addr Size Offset objfile
0x8000 0xb000 0x3000 0x0
/root/memset/main-fixed-memset.elf
0x12000 0x15000 0x3000 0x2000
/root/memset/main-fixed-memset.elf
0xb6fff000 0xb7000000 0x1000 0x0 [sigpage]
0xbefdf000 0xbf000000 0x21000 0x0
0xffff0000 0xffff1000 0x1000 0x0 [vectors]
(gdb) info reg $sp
sp 0x14d78 0x14d78
"""
GDB crashes inside the memset function, on the store instruction. This
time the crash is (as expected) because of a memory access imediately
after the memory region that contains the stack -- the buffer that's
being memset'd is allocated on the stack.
Signed-off-by: Lucian Cojocar <lucian.cojocar(a)vu.nl>
---
libc/string/arm/memset.S | 20 ++++++++++----------
1 file changed, 10 insertions(+), 10 deletions(-)
diff --git a/libc/string/arm/memset.S b/libc/string/arm/memset.S
index 2be4850..412270f 100644
--- a/libc/string/arm/memset.S
+++ b/libc/string/arm/memset.S
@@ -67,7 +67,7 @@ memset:
memset:
mov a4, a1
cmp a3, $8 @ at least 8 bytes to do?
- blt 2f
+ blo 2f
orr a2, a2, a2, lsl $8
orr a2, a2, a2, lsl $16
1:
@@ -84,27 +84,27 @@ memset:
mov ip, a2
1:
cmp a3, $8 @ 8 bytes still to do?
- blt 2f
+ blo 2f
stmia a4!, {a2, ip}
sub a3, a3, $8
cmp a3, $8 @ 8 bytes still to do?
- blt 2f
+ blo 2f
stmia a4!, {a2, ip}
sub a3, a3, $8
cmp a3, $8 @ 8 bytes still to do?
- blt 2f
+ blo 2f
stmia a4!, {a2, ip}
sub a3, a3, $8
cmp a3, $8 @ 8 bytes still to do?
#if defined(__thumb2__)
- itt ge
- stmiage a4!, {a2, ip}
- subge a3, a3, $8
+ itt hs
+ stmiahs a4!, {a2, ip}
+ subhs a3, a3, $8
#else
- stmgeia a4!, {a2, ip}
- subge a3, a3, $8
+ stmhsia a4!, {a2, ip}
+ subhs a3, a3, $8
#endif
- bge 1b
+ bhs 1b
2:
movs a3, a3 @ anything left?
IT(t, eq)
--
2.1.4