[PATCH 5.10.y] iomap: don't mark unwritten parts of a folio uptodate

[Sasha Levin <sashal@kernel.org>]

From: Christian Brauner <brauner@kernel.org>

[ Upstream commit a4193b4af359db84b2cdf3a040752861984c636b ]

For a buffered write into a hole, an unwritten or new extent, or past
EOF __iomap_write_begin() zeroes the parts of the blocks outside of the
write and then marks the blocks uptodate. That includes the range of the
write itself which still holds whatever was in the folio before. It only
becomes valid once the copy from userspace has filled it in.

But the copy can come up short and __iomap_write_end() only rejects a
short copy if the folio isn't uptodate. If the zeroed blocks were all
that was missing the folio is uptodate by then, the short copy is
accepted, and the rest of the range is exposed to readers and can end up
on disk. A source buffer that ends in an unmapped page is enough:

        ftruncate(fd, 4096);
        pwrite(fd, p, 3000, 100);    /* 16 bytes at p, then PROT_NONE */
        pread(fd, buf, 4096, 0);

The write returns 16 and the read returns 2984 bytes of whatever was in
that page before. If other blocks of the folio aren't uptodate the short
copy is rejected but the zeroed blocks keep their uptodate bits and the
retry with the copied length leaves the same bytes behind.

Only mark blocks uptodate in __iomap_write_begin() after reading them.
__iomap_write_end() marks the zeroed blocks uptodate once the copy has
succeeded and rejects a short copy.

Willy reported this back in 2024 (cf. [1]) and Zhang Yi posted the same
fix (cf. [2]) but it never got merged.

[1]: https://lore.kernel.org/all/ZqsN5ouQTEc1KAzV@casper.infradead.org
[2]: https://lore.kernel.org/all/20240812121159.3775074-6-yi.zhang@huaweicloud.com

Fixes: 9dc55f1389f9 ("iomap: add support for sub-pagesize buffered I/O without buffer heads")
Reported-by: Josef Bacik <josef@toxicpanda.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20261029-work-iomap-ai-fixes-v1-1-7eda63bc1b13@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
[ sashal: adapted iomap_set_range_uptodate() to use page instead of folio in the older __iomap_write_begin() context ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/iomap/buffered-io.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index d0a141816f125..8756797f224d0 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -582,14 +582,15 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
 		if (iomap_block_needs_zeroing(inode, srcmap, block_start)) {
 			if (WARN_ON_ONCE(flags & IOMAP_WRITE_F_UNSHARE))
 				return -EIO;
+			/* uptodate once copied, see __iomap_write_end() */
 			zero_user_segments(page, poff, from, to, poff + plen);
 		} else {
 			int status = iomap_read_page_sync(block_start, page,
 					poff, plen, srcmap);
 			if (status)
 				return status;
+			iomap_set_range_uptodate(page, poff, plen);
 		}
-		iomap_set_range_uptodate(page, poff, plen);
 	} while ((block_start += plen) < block_end);
 
 	return 0;
-- 
2.53.0