On Thu, Aug 26, 2021 at 12:23:37PM +0200, Pavel Machek wrote:
Hi!
As far as I can tell, CVE-2021-3600 is still a problem for 4.14 and 4.19.
Unfortunately, those kernels lack BPF_JMP32 support, and that support is too big and intrusive to backport.
So I tried to come up with solution without JMP32 support... only to end up with not seeing the bug in the affected code.
Changelog says:
bpf: Fix 32 bit src register truncation on div/modWhile reviewing a different fix, John and I noticed an oddity in one of the BPF program dumps that stood out, for example: # bpftool p d x i 13 0: (b7) r0 = 808464450 1: (b4) w4 = 808464432 2: (bc) w0 = w0 3: (15) if r0 == 0x0 goto pc+1 4: (9c) w4 %= w0 [...] In line 2 we noticed that the mov32 would 32 bit truncate the original src register for the div/mod operation. While for the two operations the dst register is typically marked unknown e.g. from adjust_scalar_min_max_vals() the src register is not, and thus verifier keeps tracking original bounds, simplified:
So this explains "mov32 w0, w0" is problematic, and fixes the bug by replacing it with jmp32. Unfortunately, I can't do that in 4.19; plus I don't really see how the bug is solved -- we avoided adding mov32 sequence that triggers the problem, but the problematic sequence could still be produced by the userspace, no?
Does adjust_scalar_min_max_vals still need fixing?
Do you have any hints how to solve this in 4.19?
Best regards, Pavel -- DENX Software Engineering GmbH, Managing Director: Wolfgang Denk HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Hi, Pavel.
I have just sent the fixes for 4.14. I sent fixes for 4.19 last Friday.
The problem here is that the verifier assumes the source register has a given value, but the fixups change that value to something else when it is truncated.
The fixups run after the verifier, so a similar sequence produced by userspace will be correctly verified, so no fixes are necessary on adjust_scalar_min_max for this specific issue. The fixed-up code is not verified again.
The challenge in providing those fixes to 4.14 and 4.19 is the absence of JMP32 in those kernels. So, AX was taken as a temporary, so it would still work on JITs.
Cascardo.