[PATCH] mm: Return early in truncate_pagecache if newsize overflows

[Wupeng Ma]

From: Ma Wupeng <mawupeng1@xxxxxxxxxx>

Our own test reports a UBSAN in truncate_pagecache:

UBSAN: Undefined behaviour in mm/truncate.c:788:9
signed integer overflow:
9223372036854775807 + 1 cannot be represented in type 'long long int'

Call Trace:
  truncate_pagecache+0xd4/0xe0
  truncate_setsize+0x70/0x88
  simple_setattr+0xdc/0x100
  notify_change+0x654/0xb00
  do_truncate+0x108/0x1a8
  do_sys_ftruncate+0x2ec/0x4a0
  __arm64_sys_ftruncate+0x5c/0x80

For huge file which pass LONG_MAX to ftruncate, truncate_pagecache() will
be called to truncate with newsize be LONG_MAX which will lead to
overflow for holebegin:

  loff_t holebegin = round_up(newsize, PAGE_SIZE);

Since there is no meaning to truncate a file to LONG_MAX, return here
to avoid burn a bunch of cpu cycles.

Signed-off-by: Ma Wupeng <mawupeng1@xxxxxxxxxx>
---
 mm/truncate.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/truncate.c b/mm/truncate.c
index 7b4ea4c4a46b..99b6ce2d669b 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -730,6 +730,9 @@ void truncate_pagecache(struct inode *inode, loff_t newsize)
 	struct address_space *mapping = inode->i_mapping;
 	loff_t holebegin = round_up(newsize, PAGE_SIZE);
 
+	if (holebegin < 0)
+		return;
+
 	/*
 	 * unmap_mapping_range is called twice, first simply for
 	 * efficiency so that truncate_inode_pages does fewer
-- 
2.25.1