Re: [uclibc-ng-devel] uClibc-ng and uClibc memset bug, ARM

Hi all, in libc/string/arm/memset.S[0]. If the code is compiled with #undef __thumb2__ and with #undef THUMB1_ONLY (this seems to be case for Tomato[1] at least and for buildroot) then the code looks like this[2]: """ memset: mov a4, a1 cmp a3, $8 @ at least 8 bytes to do? blt 2f orr a2, a2, a2, lsl $8 orr a2, a2, a2, lsl $16 ... 2: movs a3, a3 @ anything left? IT(t, eq) BXC(eq, lr) @ nope rsb a3, a3, $7 add pc, pc, a3, lsl $2 <--- a3 can be larger than $7 here mov r0, r0 strb a2, [a4], $1 strb a2, [a4], $1 ... """" The problem is that 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. In short, an attacker gains control of PC through the len parameter of memset. The attack is a bit unrealistic, as it requires that the application that uses uClibc allows a user to control a memory chunk larger than 2GB. I only tested this on qemu-system-arm[3]. The code was just calling memset(buf, 0xaa, 0xffff0000), memset, in this example[3] is @0x1003c. This bug is similar to CVE-2011-2702[4, 5]. Probably we should notify oss-security and get a CVE for this as the impact is unknown.