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

Kees Cook
2024-05-13T20:01:59Z

On Thu, May 09, 2024 at 11:42:07PM +0000, Justin Stitt wrote:
> 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;

This seems effectively unchanged compared to v1?

https://lore.kernel.org/all/CAFhGd8qbUYXmgiFuLGQ7dWXFUtZacvT82wD4jSS-xNTvtzXKGQ@mail.gmail.com/

> 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;

I like these changes -- they make this much more readable.

>
> /* 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;

Yeah, this is a good error code change. This is ultimately exposed via
copy_file_range, where this error is documented as possible.

-Kees

--
Kees Cook