linux-hardening - [PATCH] fs: fix unintentional arithmetic wraparound in offset calculation

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Date: Thu, 09 May 2024 21:34:58 +0000
From: Justin Stitt <justinstitt@...gle.com>
To: Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Nathan Chancellor <nathan@...nel.org>, Bill Wendling <morbo@...gle.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	llvm@...ts.linux.dev, linux-hardening@...r.kernel.org, 
	Justin Stitt <justinstitt@...gle.com>
Subject: [PATCH] fs: fix unintentional arithmetic wraparound in offset calculation

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

[   67.991989] ------------[ cut here ]------------
[   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
...

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").

Since @offset is later limited by @maxsize, we can proactively safeguard
against exceeding that value and also dodge some accidental overflow
(which may cause bad file access):

	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@...r.kernel.org
Signed-off-by: Justin Stitt <justinstitt@...gle.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 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index d4c036e82b6c..10c3eaa5ef55 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_t(loff_t, offset, maxsize - eof) + eof;
 		break;
 	case SEEK_CUR:
 		/*

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

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