[PATCH 35/46] mm: fix unmap_mapping_range() beyond 4 GiB on 32-bit

[David Hildenbrand (Arm)]

From: Josef Bacik <josef at toxicpanda.com>

Commit 9eab0421fa94 ("mm: fix unmap_mapping_range high bits shift bug")
made holebegin and holelen unsigned before they are shifted, by casting
them to pgoff_t. That is an unsigned long: on a 32-bit kernel the cast
cuts off the upper half of the two loff_t values, so a hole which begins
at 4 GiB or beyond is taken for one in the first 4 GiB of the file.

truncate_pagecache() passes the new size of the file and asks for private
COWed pages to be unmapped as well. If a file of 6 GiB is truncated to
5 GiB, everything from 1 GiB on is unmapped, and what a process has
written to a private mapping of the range between 1 GiB and 5 GiB is
lost: it reads the contents of the file again.

Where the range has an end, the wrong range is unmapped instead of the
right one. The page cache copes with that, as it unmaps every folio that
it removes. The callers which only have unmap_mapping_range() to get
rid of user mappings do not, for example drm_vma_node_unmap(), whose fake
offsets reach beyond 4 GiB on 32-bit, and revoke_iomem() with physical
addresses of more than 32 bits.

Cast to u64 instead. Nothing changes for 64-bit kernels.

Fix generated by an LLM and sent by Josef Bacik in private.

Fixes: 9eab0421fa94 ("mm: fix unmap_mapping_range high bits shift bug")
Cc: stable at vger.kernel.org
Signed-off-by: Josef Bacik <josef at toxicpanda.com>
Acked-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>
---
 mm/memory.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index ec7a1397ab833..5c5f326b8bcbe 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4649,8 +4649,8 @@ EXPORT_SYMBOL_GPL(unmap_mapping_pages);
 void unmap_mapping_range(struct address_space *mapping,
 		loff_t const holebegin, loff_t const holelen, int even_cows)
 {
-	pgoff_t hba = (pgoff_t)(holebegin) >> PAGE_SHIFT;
-	pgoff_t hlen = ((pgoff_t)(holelen) + PAGE_SIZE - 1) >> PAGE_SHIFT;
+	pgoff_t hba = (u64)holebegin >> PAGE_SHIFT;
+	pgoff_t hlen = ((u64)holelen + PAGE_SIZE - 1) >> PAGE_SHIFT;
 
 	/* Check for overflow. */
 	if (sizeof(holelen) > sizeof(hlen)) {

-- 
2.43.0