[PATCH v2] fs: fix unintentional arithmetic wraparound in offset calculation

Justin Stitt
2024-05-09T23:42:09Z

When running syzkaller with the newly reintroduced signed integer
overflow sanitizer we encounter this report:

[ 67.995501] UBSAN: signed-integer-overflow in ../fs/read_write.c:91:10
[ 68.000067] 9223372036854775807 + 4096 cannot be represented in type 'loff_t' (aka 'long long')
[ 68.006266] CPU: 4 PID: 10851 Comm: syz-executor.5 Not tainted 6.8.0-rc2-00035-gb3ef86b5a957 #1
[ 68.012353] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[ 68.018983] Call Trace:
[ 68.020803] <TASK>
[ 68.022540] dump_stack_lvl+0x93/0xd0
[ 68.025222] handle_overflow+0x171/0x1b0
[ 68.028053] generic_file_llseek_size+0x35b/0x380

amongst others:
UBSAN: signed-integer-overflow in ../fs/read_write.c:1657:12
142606336 - -9223372036854775807 cannot be represented in type 'loff_t' (aka 'long long')
...
UBSAN: signed-integer-overflow in ../fs/read_write.c:1666:11
9223372036854775807 - -9223231299366420479 cannot be represented in type 'loff_t' (aka 'long long')

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").

Fix the accidental overflow in these position and offset calculations
by checking for negative position values, using check_add_overflow()
helpers and clamping values to expected ranges.

Since @offset is later limited by @maxsize, we can proactively safeguard
against exceeding that value (and by extension avoiding integer overflow):
loff_t vfs_setpos(struct file *file, loff_t offset, loff_t maxsize)
{
if (offset < 0 && !unsigned_offsets(file))
return -EINVAL;
if (offset > maxsize)
return -EINVAL;
...

Link: https://github.com/llvm/llvm-project/pull/82432 [1]
Closes: https://github.com/KSPP/linux/issues/358
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Changes in v2:
- fix some more cases syzkaller found in read_write.c
- use min over min_t as the types are the same
- Link to v1: https://lore.kernel.org/r/20240509-b4-sio-read_write-v1-1-06bec2022697@google.com
---
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/address_bits', 0x0, 0x98)
| lseek(r0, 0x7fffffffffffffff, 0x2)

... 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/read_write.c | 18 +++++++++++-------
fs/remap_range.c | 12 ++++++------
2 files changed, 17 insertions(+), 13 deletions(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index d4c036e82b6c..d116e6e3eb3d 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -88,7 +88,7 @@ generic_file_llseek_size(struct file *file, loff_t offset, int whence,
{
switch (whence) {
case SEEK_END:
- offset += eof;
+ offset = min(offset, maxsize - eof) + eof;
break;
case SEEK_CUR:
/*
@@ -105,7 +105,8 @@ generic_file_llseek_size(struct file *file, loff_t offset, int whence,
* like SEEK_SET.
*/
spin_lock(&file->f_lock);
- offset = vfs_setpos(file, file->f_pos + offset, maxsize);
+ offset = vfs_setpos(file, min(file->f_pos, maxsize - offset) +
+ offset, maxsize);
spin_unlock(&file->f_lock);
return offset;
case SEEK_DATA:
@@ -1416,7 +1417,7 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
struct inode *inode_in = file_inode(file_in);
struct inode *inode_out = file_inode(file_out);
uint64_t count = *req_count;
- loff_t size_in;
+ loff_t size_in, in_sum, out_sum;
int ret;

ret = generic_file_rw_checks(file_in, file_out);
@@ -1450,8 +1451,8 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
if (IS_SWAPFILE(inode_in) || IS_SWAPFILE(inode_out))
return -ETXTBSY;

- /* Ensure offsets don't wrap. */
- if (pos_in + count < pos_in || pos_out + count < pos_out)
+ if (check_add_overflow(pos_in, count, &in_sum) ||
+ check_add_overflow(pos_out, count, &out_sum))
return -EOVERFLOW;

/* Shorten the copy to EOF */
@@ -1467,8 +1468,8 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,

/* Don't allow overlapped copying within the same file. */
if (inode_in == inode_out &&
- pos_out + count > pos_in &&
- pos_out < pos_in + count)
+ out_sum > pos_in &&
+ pos_out < in_sum)
return -EINVAL;

*req_count = count;
@@ -1649,6 +1650,9 @@ int generic_write_check_limits(struct file *file, loff_t pos, loff_t *count)
loff_t max_size = inode->i_sb->s_maxbytes;
loff_t limit = rlimit(RLIMIT_FSIZE);

+ if (pos < 0)
+ return -EINVAL;
+
if (limit != RLIM_INFINITY) {
if (pos >= limit) {
send_sig(SIGXFSZ, current, 0);
diff --git a/fs/remap_range.c b/fs/remap_range.c
index de07f978ce3e..4570be4ef463 100644
--- a/fs/remap_range.c
+++ b/fs/remap_range.c
@@ -36,7 +36,7 @@ static int generic_remap_checks(struct file *file_in, loff_t pos_in,
struct inode *inode_out = file_out->f_mapping->host;
uint64_t count = *req_count;
uint64_t bcount;
- loff_t size_in, size_out;
+ loff_t size_in, size_out, in_sum, out_sum;
loff_t bs = inode_out->i_sb->s_blocksize;
int ret;

@@ -44,17 +44,17 @@ static int generic_remap_checks(struct file *file_in, loff_t pos_in,
if (!IS_ALIGNED(pos_in, bs) || !IS_ALIGNED(pos_out, bs))
return -EINVAL;

- /* Ensure offsets don't wrap. */
- if (pos_in + count < pos_in || pos_out + count < pos_out)
- return -EINVAL;
+ if (check_add_overflow(pos_in, count, &in_sum) ||
+ check_add_overflow(pos_out, count, &out_sum))
+ return -EOVERFLOW;

size_in = i_size_read(inode_in);
size_out = i_size_read(inode_out);

/* Dedupe requires both ranges to be within EOF. */
if ((remap_flags & REMAP_FILE_DEDUP) &&
- (pos_in >= size_in || pos_in + count > size_in ||
- pos_out >= size_out || pos_out + count > size_out))
+ (pos_in >= size_in || in_sum > size_in ||
+ pos_out >= size_out || out_sum > size_out))
return -EINVAL;

/* Ensure the infile range is within the infile. */

---
base-commit: 0106679839f7c69632b3b9833c3268c316c0a9fc
change-id: 20240509-b4-sio-read_write-04a17d40620e

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