| Message ID | 20220223155918.927140-1-willy@infradead.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/filemap: Fix handling of THPs in generic_file_buffered_read() | expand |
On 2/23/22 16:59, Matthew Wilcox (Oracle) wrote: > When a THP is present in the page cache, we can return it several times, > leading to userspace seeing the same data repeatedly if doing a read() > that crosses a 64-page boundary. This is probably not a security issue > (since the data all comes from the same file), but it can be interpreted > as a transient data corruption issue. Fortunately, it is very rare as > it can only occur when CONFIG_READ_ONLY_THP_FOR_FS is enabled, and it can > only happen to executables. We don't often call read() on executables. > > This bug is fixed differently in v5.17 by commit 6b24ca4a1a8d > ("mm: Use multi-index entries in the page cache"). That commit is > unsuitable for backporting, so fix this in the clearest way. It > sacrifices a little performance for clarity, but this should never > be a performance path in these kernel versions. > > Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read") > Cc: stable@vger.kernel.org # v5.15, v5.16 > Link: https://lore.kernel.org/r/df3b5d1c-a36b-2c73-3e27-99e74983de3a@suse.cz/ > Analyzed-by: Adam Majer <amajer@suse.com> > Analyzed-by: Dirk Mueller <dmueller@suse.com> > Bisected-by: Takashi Iwai <tiwai@suse.de> > Reported-by: Vlastimil Babka <vbabka@suse.cz> Replace with: Reported-and-tested-by: Vlastimil Babka <vbabka@suse.cz> Thanks! > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> > --- > mm/filemap.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/mm/filemap.c b/mm/filemap.c > index 82a17c35eb96..1293c3409e42 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -2354,8 +2354,12 @@ static void filemap_get_read_batch(struct address_space *mapping, > break; > if (PageReadahead(head)) > break; > - xas.xa_index = head->index + thp_nr_pages(head) - 1; > - xas.xa_offset = (xas.xa_index >> xas.xa_shift) & XA_CHUNK_MASK; > + if (PageHead(head)) { > + xas_set(&xas, head->index + thp_nr_pages(head)); > + /* Handle wrap correctly */ > + if (xas.xa_index - 1 >= max) > + break; > + } > continue; > put_page: > put_page(head);
On Wed, Feb 23, 2022 at 03:59:18PM +0000, Matthew Wilcox (Oracle) wrote: > When a THP is present in the page cache, we can return it several times, > leading to userspace seeing the same data repeatedly if doing a read() > that crosses a 64-page boundary. This is probably not a security issue > (since the data all comes from the same file), but it can be interpreted > as a transient data corruption issue. Fortunately, it is very rare as > it can only occur when CONFIG_READ_ONLY_THP_FOR_FS is enabled, and it can > only happen to executables. We don't often call read() on executables. > > This bug is fixed differently in v5.17 by commit 6b24ca4a1a8d > ("mm: Use multi-index entries in the page cache"). That commit is > unsuitable for backporting, so fix this in the clearest way. It > sacrifices a little performance for clarity, but this should never > be a performance path in these kernel versions. > > Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read") > Cc: stable@vger.kernel.org # v5.15, v5.16 > Link: https://lore.kernel.org/r/df3b5d1c-a36b-2c73-3e27-99e74983de3a@suse.cz/ > Analyzed-by: Adam Majer <amajer@suse.com> > Analyzed-by: Dirk Mueller <dmueller@suse.com> > Bisected-by: Takashi Iwai <tiwai@suse.de> > Reported-by: Vlastimil Babka <vbabka@suse.cz> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> > --- > mm/filemap.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/mm/filemap.c b/mm/filemap.c > index 82a17c35eb96..1293c3409e42 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -2354,8 +2354,12 @@ static void filemap_get_read_batch(struct address_space *mapping, > break; > if (PageReadahead(head)) > break; > - xas.xa_index = head->index + thp_nr_pages(head) - 1; > - xas.xa_offset = (xas.xa_index >> xas.xa_shift) & XA_CHUNK_MASK; > + if (PageHead(head)) { > + xas_set(&xas, head->index + thp_nr_pages(head)); > + /* Handle wrap correctly */ > + if (xas.xa_index - 1 >= max) > + break; > + } > continue; > put_page: > put_page(head); > -- > 2.34.1 > Now queued up, thanks! greg k-h
diff --git a/mm/filemap.c b/mm/filemap.c index 82a17c35eb96..1293c3409e42 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2354,8 +2354,12 @@ static void filemap_get_read_batch(struct address_space *mapping, break; if (PageReadahead(head)) break; - xas.xa_index = head->index + thp_nr_pages(head) - 1; - xas.xa_offset = (xas.xa_index >> xas.xa_shift) & XA_CHUNK_MASK; + if (PageHead(head)) { + xas_set(&xas, head->index + thp_nr_pages(head)); + /* Handle wrap correctly */ + if (xas.xa_index - 1 >= max) + break; + } continue; put_page: put_page(head);
When a THP is present in the page cache, we can return it several times, leading to userspace seeing the same data repeatedly if doing a read() that crosses a 64-page boundary. This is probably not a security issue (since the data all comes from the same file), but it can be interpreted as a transient data corruption issue. Fortunately, it is very rare as it can only occur when CONFIG_READ_ONLY_THP_FOR_FS is enabled, and it can only happen to executables. We don't often call read() on executables. This bug is fixed differently in v5.17 by commit 6b24ca4a1a8d ("mm: Use multi-index entries in the page cache"). That commit is unsuitable for backporting, so fix this in the clearest way. It sacrifices a little performance for clarity, but this should never be a performance path in these kernel versions. Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read") Cc: stable@vger.kernel.org # v5.15, v5.16 Link: https://lore.kernel.org/r/df3b5d1c-a36b-2c73-3e27-99e74983de3a@suse.cz/ Analyzed-by: Adam Majer <amajer@suse.com> Analyzed-by: Dirk Mueller <dmueller@suse.com> Bisected-by: Takashi Iwai <tiwai@suse.de> Reported-by: Vlastimil Babka <vbabka@suse.cz> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> --- mm/filemap.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)