The quilt patch titled Subject: writeback: fix dereferencing NULL mapping->host on writeback_page_template has been removed from the -mm tree. Its filename was writeback-fix-dereferencing-null-mapping-host-on-writeback_page_template.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------ From: Rafael Aquini aquini@redhat.com Subject: writeback: fix dereferencing NULL mapping->host on writeback_page_template Date: Tue, 6 Jun 2023 19:36:13 -0400

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 ---

include/trace/events/writeback.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/include/trace/events/writeback.h~writeback-fix-dereferencing-null-mapping-host-on-writeback_page_template +++ a/include/trace/events/writeback.h @@ -68,7 +68,7 @@ DECLARE_EVENT_CLASS(writeback_folio_temp strscpy_pad(__entry->name, bdi_dev_name(mapping ? inode_to_bdi(mapping->host) : NULL), 32); - __entry->ino = mapping ? mapping->host->i_ino : 0; + __entry->ino = (mapping && mapping->host) ? mapping->host->i_ino : 0; __entry->index = folio->index; ),

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Patches currently in -mm which might be from aquini@redhat.com are