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/mod > > While 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.
