[PATCH] fs: remove accidental overflow during wraparound check

[Justin Stitt]

Running syzkaller with the newly enabled signed integer overflow
sanitizer produces this report:

[  195.401651] ------------[ cut here ]------------
[  195.404808] UBSAN: signed-integer-overflow in ../fs/open.c:321:15
[  195.408739] 9223372036854775807 + 562984447377399 cannot be represented in type 'loff_t' (aka 'long long')
[  195.414683] CPU: 1 PID: 703 Comm: syz-executor.0 Not tainted 6.8.0-rc2-00039-g14de58dbe653-dirty #11
[  195.420138] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[  195.425804] Call Trace:
[  195.427360]  <TASK>
[  195.428791]  dump_stack_lvl+0x93/0xd0
[  195.431150]  handle_overflow+0x171/0x1b0
[  195.433640]  vfs_fallocate+0x459/0x4f0
..
[  195.490053] ------------[ cut here ]------------
[  195.493146] UBSAN: signed-integer-overflow in ../fs/open.c:321:61
[  195.497030] 9223372036854775807 + 562984447377399 cannot be represented in type 'loff_t' (aka 'long long)
[  195.502940] CPU: 1 PID: 703 Comm: syz-executor.0 Not tainted 6.8.0-rc2-00039-g14de58dbe653-dirty #11
[  195.508395] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[  195.514075] Call Trace:
[  195.515636]  <TASK>
[  195.517000]  dump_stack_lvl+0x93/0xd0
[  195.519255]  handle_overflow+0x171/0x1b0
[  195.521677]  vfs_fallocate+0x4cb/0x4f0
[  195.524033]  __x64_sys_fallocate+0xb2/0xf0

Historically, the signed integer overflow sanitizer did not work in the
kernel due to its interaction with `-fwrapv` but this has since been
changed [1] in the newest version of Clang. It was re-enabled in the
kernel with Commit 557f8c582a9ba8ab ("ubsan: Reintroduce signed overflow
sanitizer").

Let's use the check_add_overflow helper to first verify the addition
stays within the bounds of its type (long long); then we can use that
sum for the following check.

Link: https://github.com/llvm/llvm-project/pull/82432 [1]
Closes: https://github.com/KSPP/linux/issues/356
Cc: linux-hardening@xxxxxxxxxxxxxxx
Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
---
I wonder, though, why isn't loff_t an unsigned type? We have plently of
checks to ensure they are positive:

	if (offset < 0 || len <= 0)
		return -EINVAL;
	...
	if (((offset + len) > inode->i_sb->s_maxbytes) || ((offset + len) < 0))

.. are there ABI concerns?

Here's the syzkaller reproducer:
r0 = openat(0xffffffffffffff9c, &(0x7f0000000040)='./file1\x00', 0x42, 0x0)
fallocate(r0, 0x10, 0x7fffffffffffffff, 0x2000807fffff7)

.. which was used against Kees' tree here (v6.8rc2):
https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=wip/v6.9-rc2/unsigned-overflow-sanitizer

.. with this config:
https://gist.github.com/JustinStitt/824976568b0f228ccbcbe49f3dee9bf4
---
 fs/open.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index ee8460c83c77..d216e69d6872 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -247,6 +247,7 @@ int vfs_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
 {
 	struct inode *inode = file_inode(file);
 	long ret;
+	loff_t sum;
 
 	if (offset < 0 || len <= 0)
 		return -EINVAL;
@@ -319,8 +320,12 @@ int vfs_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
 	if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))
 		return -ENODEV;
 
-	/* Check for wrap through zero too */
-	if (((offset + len) > inode->i_sb->s_maxbytes) || ((offset + len) < 0))
+	/* Check for wraparound */
+	if (check_add_overflow(offset, len, &sum))
+		return -EFBIG;
+
+	/* Now, check bounds */
+	if (sum > inode->i_sb->s_maxbytes || sum < 0)
 		return -EFBIG;
 
 	if (!file->f_op->fallocate)

---
base-commit: 0106679839f7c69632b3b9833c3268c316c0a9fc
change-id: 20240507-b4-sio-vfs_fallocate-7b5223ba3a81

Best regards,
--
Justin Stitt <justinstitt@xxxxxxxxxx>