From a4193b4af359db84b2cdf3a040752861984c636b Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Thu, 29 Oct 2026 11:34:48 +0100 Subject: [PATCH] iomap: don't mark unwritten parts of a folio uptodate 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 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) --- 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 678f4c5329e7..a6ab6f31cbbf 100644 --- a/fs/iomap/buffered-io.c +++ b/fs/iomap/buffered-io.c @@ -920,6 +920,7 @@ static int __iomap_write_begin(const struct iomap_iter *iter, if (iomap_block_needs_zeroing(iter, block_start)) { if (WARN_ON_ONCE(iter->flags & IOMAP_UNSHARE)) return -EIO; + /* uptodate once copied, see __iomap_write_end() */ folio_zero_segments(folio, poff, from, to, poff + plen); } else { const struct iomap *iomap = iomap_iter_srcmap(iter); @@ -943,8 +944,8 @@ static int __iomap_write_begin(const struct iomap_iter *iter, if (iomap->flags & IOMAP_F_ZERO_TAIL) folio_zero_segment(folio, to, poff + plen); + iomap_set_range_uptodate(folio, poff, plen); } - iomap_set_range_uptodate(folio, poff, plen); } while ((block_start += plen) < block_end); return 0;