Re: [PATCH] writeback: fix dereferencing NULL mapping->host on writeback_page_template

[Rafael Aquini]

On Tue, Jun 06, 2023 at 05:44:48PM -0700, Andrew Morton wrote:
> On Tue,  6 Jun 2023 19:36:13 -0400 Rafael Aquini <aquini@xxxxxxxxxx> wrote:
> 
> > When commit 19343b5bdd16 ("mm/page-writeback: introduce tracepoint for
> > wait_on_page_writeback()") repurposed the writeback_dirty_page trace event
> > as a template to create its new wait_on_page_writeback trace event, it
> > ended up opening a window to NULL pointer dereference crashes due to
> > the (infrequent) occurrence of a race where an access to a page in the
> > swap-cache happens concurrently with the moment this page is being
> > written to disk and the tracepoint is enabled:
> 
> I don't see what the race is, or why a race is involved.
> 
> >     BUG: kernel NULL pointer dereference, address: 0000000000000040
> >     #PF: supervisor read access in kernel mode
> >     #PF: error_code(0x0000) - not-present page
> >     PGD 800000010ec0a067 P4D 800000010ec0a067 PUD 102353067 PMD 0
> >     Oops: 0000 [#1] PREEMPT SMP PTI
> >     CPU: 1 PID: 1320 Comm: shmem-worker Kdump: loaded Not tainted 6.4.0-rc5+ #13
> >     Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS edk2-20230301gitf80f052277c8-1.fc37 03/01/2023
> >     RIP: 0010:trace_event_raw_event_writeback_folio_template+0x76/0xf0
> >     Code: 4d 85 e4 74 5c 49 8b 3c 24 e8 06 98 ee ff 48 89 c7 e8 9e 8b ee ff ba 20 00 00 00 48 89 ef 48 89 c6 e8 fe d4 1a 00 49 8b 04 24 <48> 8b 40 40 48 89 43 28 49 8b 45 20 48 89 e7 48 89 43 30 e8 a2 4d
> >     RSP: 0000:ffffaad580b6fb60 EFLAGS: 00010246
> >     RAX: 0000000000000000 RBX: ffff90e38035c01c RCX: 0000000000000000
> >     RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff90e38035c044
> >     RBP: ffff90e38035c024 R08: 0000000000000002 R09: 0000000000000006
> >     R10: ffff90e38035c02e R11: 0000000000000020 R12: ffff90e380bac000
> >     R13: ffffe3a7456d9200 R14: 0000000000001b81 R15: ffffe3a7456d9200
> >     FS:  00007f2e4e8a15c0(0000) GS:ffff90e3fbc80000(0000) knlGS:0000000000000000
> >     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >     CR2: 0000000000000040 CR3: 00000001150c6003 CR4: 0000000000170ee0
> >     Call Trace:
> >      <TASK>
> >      ? __die+0x20/0x70
> >      ? page_fault_oops+0x76/0x170
> >      ? kernelmode_fixup_or_oops+0x84/0x110
> >      ? exc_page_fault+0x65/0x150
> >      ? asm_exc_page_fault+0x22/0x30
> >      ? trace_event_raw_event_writeback_folio_template+0x76/0xf0
> >      folio_wait_writeback+0x6b/0x80
> >      shmem_swapin_folio+0x24a/0x500
> 
> shmem_swapin_folio->folio_wait_writeback will always pass in a page
> which has ->mapping==NULL, won't it?  So why doesn't it crash every
> time?
>

Hey Andrew,

Here's why we end up looking at the swapper_spaces[] address space, for
this particular case:

void folio_wait_writeback(struct folio *folio)
{
    while (folio_test_writeback(folio)) {
        trace_folio_wait_writeback(folio, folio_mapping(folio));
            struct address_space *folio_mapping(struct folio *folio)
            ... 
                if (unlikely(folio_test_swapcache(folio)))
                    return swap_address_space(folio_swap_entry(folio));

when the shmem swap-in path stumbles on a page in the swapcache that is still
under its way to disk (via swap_writepage->swap_writepage_bdev_async->submit_bio)
the tracepoint, will get a swap_address_space pointer back from folio_mapping()