Messages in this thread | | | Subject | Re: [PATCH 5.4 10/36] bpf: Fix tnum constraints for 32-bit comparisons | From | Daniel Borkmann <> | Date | Tue, 7 Apr 2020 12:45:23 +0200 |
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Hey Sasha, hey Greg,
On 4/7/20 12:21 PM, Greg Kroah-Hartman wrote: > From: Jann Horn <jannh@google.com> > > [ Upstream commit 604dca5e3af1db98bd123b7bfc02b017af99e3a0 ] > > The BPF verifier tried to track values based on 32-bit comparisons by > (ab)using the tnum state via 581738a681b6 ("bpf: Provide better register > bounds after jmp32 instructions"). The idea is that after a check like > this: > > if ((u32)r0 > 3) > exit > > We can't meaningfully constrain the arithmetic-range-based tracking, but > we can update the tnum state to (value=0,mask=0xffff'ffff'0000'0003). > However, the implementation from 581738a681b6 didn't compute the tnum > constraint based on the fixed operand, but instead derives it from the > arithmetic-range-based tracking. This means that after the following > sequence of operations: > > if (r0 >= 0x1'0000'0001) > exit > if ((u32)r0 > 7) > exit > > The verifier assumed that the lower half of r0 is in the range (0, 0) > and apply the tnum constraint (value=0,mask=0xffff'ffff'0000'0000) thus > causing the overall tnum to be (value=0,mask=0x1'0000'0000), which was > incorrect. Provide a fixed implementation. > > Fixes: 581738a681b6 ("bpf: Provide better register bounds after jmp32 instructions") > Signed-off-by: Jann Horn <jannh@google.com> > Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> > Signed-off-by: Alexei Starovoitov <ast@kernel.org> > Link: https://lore.kernel.org/bpf/20200330160324.15259-3-daniel@iogearbox.net > Signed-off-by: Sasha Levin <sashal@kernel.org>
We've already addressed this issue (CVE-2020-8835) on 5.4/5.5/5.6 kernels through the following backports:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-5.4.y&id=8d62a8c7489a68b5738390b008134a644aa9b383 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-5.5.y&id=0ebc01466d98d016eb6a3780ec8edb0c86fa48bc https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-5.6.y&id=6797143df51c8ae259aa4bfe4e99c832b20bde8a
Given the severity of the issue, we concluded that revert-only is the best and most straight forward way to address it for stable.
Was this selected via Sasha's ML mechanism? Should there be a commit tag to opt-out for some commits being selected? E.g. this one 581738a681b6 ("bpf: Provide better register bounds after jmp32 instructions") already fell through our radar and wrongly made its way into 5.4 where it should have never landed. :/
Thanks, Daniel
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