Re: [PATCH 5.4.y] writeback: fix dereferencing NULL mapping->host on writeback_page_template

[Rafael Aquini <aquini@redhat.com>]

On Sat, Jun 24, 2023 at 04:04:38PM +0200, Greg KH wrote:
> On Fri, Jun 23, 2023 at 09:46:01AM -0400, Rafael Aquini wrote:
> > commit 54abe19e00cfcc5a72773d15cd00ed19ab763439 upstream.
> > 
> > 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:
> > 
> >     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
> >      ? filemap_get_entry+0xe3/0x140
> >      shmem_get_folio_gfp+0x36e/0x7c0
> >      ? find_busiest_group+0x43/0x1a0
> >      shmem_fault+0x76/0x2a0
> >      ? __update_load_avg_cfs_rq+0x281/0x2f0
> >      __do_fault+0x33/0x130
> >      do_read_fault+0x118/0x160
> >      do_pte_missing+0x1ed/0x2a0
> >      __handle_mm_fault+0x566/0x630
> >      handle_mm_fault+0x91/0x210
> >      do_user_addr_fault+0x22c/0x740
> >      exc_page_fault+0x65/0x150
> >      asm_exc_page_fault+0x22/0x30
> > 
> > This problem arises from the fact that the repurposed writeback_dirty_page
> > trace event code was written assuming that every pointer to mapping
> > (struct address_space) would come from a file-mapped page-cache object,
> > thus mapping->host would always be populated, and that was a valid case
> > before commit 19343b5bdd16.  The swap-cache address space
> > (swapper_spaces), however, doesn't populate its ->host (struct inode)
> > pointer, thus leading to the crashes in the corner-case aforementioned.
> > 
> > commit 19343b5bdd16 ended up breaking the assignment of __entry->name and
> > __entry->ino for the wait_on_page_writeback tracepoint -- both dependent
> > on mapping->host carrying a pointer to a valid inode.  The assignment of
> > __entry->name was fixed by commit 68f23b89067f ("memcg: fix a crash in
> > wb_workfn when a device disappears"), and this commit fixes the remaining
> > case, for __entry->ino.
> > 
> > Link: https://lkml.kernel.org/r/20230606233613.1290819-1-aquini@redhat.com
> > Fixes: 19343b5bdd16 ("mm/page-writeback: introduce tracepoint for wait_on_page_writeback()")
> > Signed-off-by: Rafael Aquini <aquini@redhat.com>
> > Reviewed-by: Yafang Shao <laoar.shao@gmail.com>
> > Cc: Aristeu Rozanski <aris@redhat.com>
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> > Signed-off-by: Rafael Aquini <aquini@redhat.com>
> > ---
> >  include/trace/events/writeback.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> All now queued up, thanks.
>

Thank you, Greg.

-- Rafael