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@xxxxxxxxxxxxxxx Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx> --- 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@xxxxxxxxxx>
