From 8f9039840588dd618c72a726c192126bb7b98228 Mon Sep 17 00:00:00 2001 From: Nickolai Zeldovich Date: Fri, 16 Oct 2026 18:27:21 -0400 Subject: [PATCH] mm/truncate: fix the zeroing end of pagecache_isize_extended() A process that extends a file by 4 GiB or more past a dirty folio, on a file system whose block size is smaller than the page size (ext4 with 1k blocks), reads back the bytes it wrote through a shared mapping beyond the old end of file instead of zeroes, and a later write into that block carries them to the disk. With the example of commit 52aecaee1c26 ("mm: zero range of eof folio exposed by inode size extension") and the truncate taken 4 GiB further (pwrite 2k at 0, mmap the first page and write 'X' over [2k, 4k), truncate(2) to 4 GiB + 2k), the read of [2k, 4k) returns the 2048 'X' bytes, and a pwrite of 16 bytes at 3k followed by fsync() and a remount leaves 1008 'X' bytes on the disk in [3k+16, 4k). No privilege is needed, and every size extension of 4 GiB or more reaches it: truncate(2)/ftruncate(2) through truncate_setsize(), writes beyond the end of file through generic_write_end(), ext4, iomap, btrfs, exfat, ntfs, afs and orangefs. Affected since v6.13. pagecache_isize_extended() zeroes the part of the dirty folio straddling the old end of file that the size extension exposes, from the old size to the new size or the end of the folio, whichever comes first: end = min_t(unsigned int, to - folio_pos(folio), folio_size(folio)); min_t() converts both operands to unsigned int before comparing, so the loff_t difference to - folio_pos(folio) is reduced modulo 2^32 first. When the file grows by 4 GiB or more past the folio, end is that residue instead of folio_size(folio): below offset nothing is zeroed, above it only [offset, end) is. The bytes beyond the old end of file stay in the dirty folio and, now inside i_size, are file data. Compare in loff_t; the result is at most folio_size(folio) and fits end. Found while formally verifying pagecache_isize_extended() against the kernel binary. Link: https://lore.kernel.org/20261016222721.791785-1-nickolai@csail.mit.edu Fixes: 52aecaee1c26 ("mm: zero range of eof folio exposed by inode size extension") Signed-off-by: Nickolai Zeldovich Signed-off-by: Andrew Morton Reviewed-by: Jan Kara Reviewed-by: Baokun Li Assisted-by: LLM Cc: Matthew Wilcox (Oracle) Cc: Brian Foster Cc: Theodore Ts'o Cc: --- mm/truncate.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/mm/truncate.c b/mm/truncate.c index 5b1b13cf8ff0..733d2713a875 100644 --- a/mm/truncate.c +++ b/mm/truncate.c @@ -919,8 +919,7 @@ void pagecache_isize_extended(struct inode *inode, loff_t from, loff_t to) unsigned int offset, end; offset = from - folio_pos(folio); - end = min_t(unsigned int, to - folio_pos(folio), - folio_size(folio)); + end = min_t(loff_t, to - folio_pos(folio), folio_size(folio)); folio_zero_segment(folio, offset, end); }