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
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.
We have fixed this in our 4.15 kernels with help from Daniel, John, Alexei and
Salvatore. I am a little busy right now, but will make time to work on the
commits we had for 4.19. 4.19 is also affected by CVE-2021-3444, IIRC.
When I get back to it, I may try to answer some of your questions.
Cascardo.
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.
Hi!
> > 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?
> 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.
Thanks, understood.
> 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.
Yes, I got that far. I have seen the patches for 4.19 and 4.14, and
they should fix my problems. Thanks a lot for them.
Best regards,
Pavel
--
DENX Software Engineering GmbH, Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany