Messages in this thread | | | Subject | Re: [PATCH] net: core: Fix Spectre v1 vulnerability | From | "Gustavo A. R. Silva" <> | Date | Sat, 22 Dec 2018 20:53:40 -0600 |
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Hi,
On 12/22/18 8:40 PM, David Miller wrote: > From: Alexei Starovoitov <alexei.starovoitov@gmail.com> > Date: Sat, 22 Dec 2018 15:59:54 -0800 > >> On Sat, Dec 22, 2018 at 03:07:22PM -0800, David Miller wrote: >>> From: "Gustavo A. R. Silva" <gustavo@embeddedor.com> >>> Date: Fri, 21 Dec 2018 14:49:01 -0600 >>> >>>> flen is indirectly controlled by user-space, hence leading to >>>> a potential exploitation of the Spectre variant 1 vulnerability. >>>> >>>> This issue was detected with the help of Smatch: >>>> >>>> net/core/filter.c:1101 bpf_check_classic() warn: potential spectre issue 'filter' [w] >>>> >>>> Fix this by sanitizing flen before using it to index filter at line 1101: >>>> >>>> switch (filter[flen - 1].code) { >>>> >>>> and through pc at line 1040: >>>> >>>> const struct sock_filter *ftest = &filter[pc]; >>>> >>>> Notice that given that speculation windows are large, the policy is >>>> to kill the speculation on the first load and not worry if it can be >>>> completed with a dependent load/store [1]. >>>> >>>> [1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2 >>>> >>>> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com> >>> >>> BPF folks, I'll take this directly. >>> >>> Applied and queued up for -stable, thanks. >> >> hmm. what was the rush? >> I think it is unnecessary change. >> Though fp is passed initially from user space >> it's copied into kernel struct first. >> There is no way user space can force kernel to mispredict >> branch in for (pc = 0; pc < flen; pc++) loop. The following piece of code is the one that can be mispredicted, not the for loop:
1013 if (flen == 0 || flen > BPF_MAXINSNS) 1014 return false;
Instead of calling array_index_nospec() inside bpf_check_basics_ok(), I decided to place the call close to the code that could be compromised. This is when accessing filter[].
-- Gustavo
>> The change doesn't harm, but I don't think it's a good idea >> to sprinkle kernel with array_index_nospec() just because some tool >> produced a warning. > > Ok, that makes sense, I can revert. > > Just let me know. >
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