[PATCH] libfs: fix accidental overflow in offset calculation

Justin Stitt
2024-05-10T00:35:53Z

Running syzkaller with the newly reintroduced signed integer overflow
sanitizer gives this report:

[ 6008.464680] UBSAN: signed-integer-overflow in ../fs/libfs.c:149:11
[ 6008.468664] 9223372036854775807 + 16387 cannot be represented in type 'loff_t' (aka 'long long')
[ 6008.474167] CPU: 1 PID: 1214 Comm: syz-executor.0 Not tainted 6.8.0-rc2-00041-gec7cb1052e44-dirty #15
[ 6008.479662] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[ 6008.485276] Call Trace:
[ 6008.486819] <TASK>
[ 6008.488258] dump_stack_lvl+0x93/0xd0
[ 6008.490535] handle_overflow+0x171/0x1b0
[ 6008.492957] dcache_dir_lseek+0x3bf/0x3d0
...

Use the check_add_overflow() helper to gracefully check for
unintentional overflow causing wraparound in our offset calculations.

Link: https://github.com/llvm/llvm-project/pull/82432 [1]
Closes: https://github.com/KSPP/linux/issues/359
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
---

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").

Here's the syzkaller reproducer:

| # {Threaded:false Repeat:false RepeatTimes:0 Procs:1 Slowdown:1 Sandbox:
| # SandboxArg:0 Leak:false NetInjection:false NetDevices:false
| # NetReset:false Cgroups:false BinfmtMisc:false CloseFDs:false KCSAN:false
| # DevlinkPCI:false NicVF:false USB:false VhciInjection:false Wifi:false
| # IEEE802154:false Sysctl:false Swap:false UseTmpDir:false
| # HandleSegv:false Repro:false Trace:false LegacyOptions:{Collide:false
| # Fault:false FaultCall:0 FaultNth:0}}
| r0 = openat$sysfs(0xffffffffffffff9c, &(0x7f0000000000)='/sys/kernel/tracing', 0x0, 0x0)
| lseek(r0, 0x4003, 0x0)
| lseek(r0, 0x7fffffffffffffff, 0x1)

... 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/libfs.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index 3a6f2cb364f8..3fdc1aaddd45 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -147,7 +147,9 @@ loff_t dcache_dir_lseek(struct file *file, loff_t offset, int whence)
struct dentry *dentry = file->f_path.dentry;
switch (whence) {
case 1:
- offset += file->f_pos;
+ /* cannot represent offset with loff_t */
+ if (check_add_overflow(offset, file->f_pos, &offset))
+ return -EOVERFLOW;
fallthrough;
case 0:
if (offset >= 0)
@@ -422,7 +424,9 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
{
switch (whence) {
case SEEK_CUR:
- offset += file->f_pos;
+ /* cannot represent offset with loff_t */
+ if (check_add_overflow(offset, file->f_pos, &offset))
+ return -EOVERFLOW;
fallthrough;
case SEEK_SET:
if (offset >= 0)

---
base-commit: 0106679839f7c69632b3b9833c3268c316c0a9fc
change-id: 20240509-b4-sio-libfs-67947244a4a3

Best regards,
--
Justin Stitt <justinstitt@google.com>