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.
Fixes: 19343b5bdd16 ("mm/page-writeback: introduce tracepoint for wait_on_page_writeback()") Signed-off-by: Rafael Aquini aquini@redhat.com --- include/trace/events/writeback.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h index 86b2a82da546..54e353c9f919 100644 --- a/include/trace/events/writeback.h +++ b/include/trace/events/writeback.h @@ -68,7 +68,7 @@ DECLARE_EVENT_CLASS(writeback_folio_template, 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; ),
Hi,
Thanks for your patch.
FYI: kernel test robot notices the stable kernel rule is not satisfied.
Rule: 'Cc: stable@vger.kernel.org' or 'commit <sha1> upstream.' Subject: [PATCH] writeback: fix dereferencing NULL mapping->host on writeback_page_template Link: https://lore.kernel.org/stable/20230606233613.1290819-1-aquini%40redhat.com
The check is based on https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
On Tue, 6 Jun 2023 19:36:13 -0400 Rafael Aquini aquini@redhat.com 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?
? 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+0x22/0x30This 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.
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@redhat.com 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/0x500shmem_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()
On Wed, Jun 7, 2023 at 7:37 AM Rafael Aquini aquini@redhat.com 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:
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+0x22/0x30This 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.
Fixes: 19343b5bdd16 ("mm/page-writeback: introduce tracepoint for wait_on_page_writeback()") Signed-off-by: Rafael Aquini aquini@redhat.com
Thanks for your fix! Reviewed-by: Yafang Shao laoar.shao@gmail.com
include/trace/events/writeback.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h index 86b2a82da546..54e353c9f919 100644 --- a/include/trace/events/writeback.h +++ b/include/trace/events/writeback.h @@ -68,7 +68,7 @@ DECLARE_EVENT_CLASS(writeback_folio_template, 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; ),-- 2.39.2
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