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SubjectRe: [BUG] from x86: Support kmap_local() forced debugging
On Wed, Jan 6, 2021 at 3:01 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> I triggered the following crash on x86_32 by simply doing a:
>
> (ssh'ing into the box)
>
> # head -100 /tmp/output-file
>
> Where the /tmp/output-file was the output of a trace-cmd report.
> Even after rebooting and not running the tracing code, simply doing the
> head command still crashed.

The code decodes to

0: 3b 5d e8 cmp -0x18(%ebp),%ebx
3: 0f 47 5d e8 cmova -0x18(%ebp),%ebx
7: c7 45 e0 00 00 00 00 movl $0x0,-0x20(%ebp)
e: 8b 7d e0 mov -0x20(%ebp),%edi
11: 39 7d e8 cmp %edi,-0x18(%ebp)
14: 76 3a jbe 0x50
16: 8b 45 d4 mov -0x2c(%ebp),%eax
19: e8 a4 e4 ff ff call 0xffffe4c2
1e: 8b 55 e4 mov -0x1c(%ebp),%edx
21: 03 55 e0 add -0x20(%ebp),%edx
24: 89 d9 mov %ebx,%ecx
26: 01 c6 add %eax,%esi
28: 89 d7 mov %edx,%edi
2a:* f3 a4 rep movsb %ds:(%esi),%es:(%edi)
<-- trapping instruction
2c: e8 c9 e4 ff ff call 0xffffe4fa
31: 01 5d e0 add %ebx,-0x20(%ebp)
34: 8b 5d e8 mov -0x18(%ebp),%ebx
37: b8 00 10 00 00 mov $0x1000,%eax
3c: 2b 5d e0 sub -0x20(%ebp),%ebx

and while it would be good to see the output of
scripts/decode_stacktrace.sh, I strongly suspect that the above is

vaddr = kmap_atomic(p);
memcpy(to + copied, vaddr + p_off, p_len);
kunmap_atomic(vaddr);

(although I wonder how/why the heck you've enabled
CC_OPTIMIZE_FOR_SIZE=y, which is what causes "memcpy()" to be done as
that "rep movsb". I thought we disabled it because it's so bad on most
cpus).

So that first "call" instruction is the kmap_atomic(), the "rep movs"
is the memcpy(), and the "call" instruction immediately after is the
kunmap_atomic().

Anyway, you can see vaddr in register state:

EAX: fff57000

so we've kmapped that one page at fff57000, but we're accessing past
it into the next page:

> BUG: unable to handle page fault for address: fff58000

with the current source address being (ESI: fff58000) and we still
have 248 bytes to go (ECX: 000000f8) even though we've already
overflowed into the next page.

You can see the original count still (EBX: 000005a8), so it really
looks like that skb_frag_foreach_page() logic

skb_frag_foreach_page(f,
skb_frag_off(f) + offset - start,
copy, p, p_off, p_len, copied) {
vaddr = kmap_atomic(p);
memcpy(to + copied, vaddr + p_off, p_len);
kunmap_atomic(vaddr);
}

must be wrong, and doesn't handle the "each page" part properly. It
must have started in the middle of the page, and p_len (that 0x5a8)
was wrong.

IOW, it really looks like p_off + p_len had the value 0x10f8, which is
larger than one page. And looking at the code, in
skb_frag_foreach_page(), I see:

p_off = (f_off) & (PAGE_SIZE - 1), \
p_len = skb_frag_must_loop(p) ? \
min_t(u32, f_len, PAGE_SIZE - p_off) : f_len, \

where that "min_t(u32, f_len, PAGE_SIZE - p_off)" looks correct, but
then presumably skb_frag_must_loop() must be wrong.

Oh, and when I look at that, I see

static inline bool skb_frag_must_loop(struct page *p)
{
#if defined(CONFIG_HIGHMEM)
if (PageHighMem(p))
return true;
#endif
return false;
}

and that is no longer true. With the kmap debugging, even non-highmem
pages need that "do one page at a time" code, because even non-highmem
pages get remapped by kmap().

IOW, I think the patch to fix this might be something like the attached.

I wonder whether there is other code that "knows" about kmap() only
affecting PageHighmem() pages thing that is no longer true.

Looking at some other code, skb_gro_reset_offset() looks suspiciously
like it also thinks highmem pages are special.

Adding the networking people involved in this area to the cc too.

Linus
[unhandled content-type:application/octet-stream]
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