Re: [PATCH] fs/sync.c: Fix UBSAN Undefined behaviour in sync_file_range

[David Sterba]

On Tue, Aug 27, 2019 at 05:22:37PM +0200, Jan Kara wrote:
> On Fri 12-07-19 10:28:37, SunKe wrote:
> > There is a UBSAN report:
> > UBSAN: Undefined behaviour in ../fs/sync.c:298:10
> > signed integer overflow:
> > -8 + -9223372036854775807 cannot be represented in type 'long long int'
> > CPU: 0 PID: 15876 Comm: syz-executor.3 Not tainted
> > Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
> > Call trace:
> > [<ffffff90080ac450>] dump_backtrace+0x0/0x698 arch/arm64/kernel/traps.c:96
> > [<ffffff90080acb20>] show_stack+0x38/0x60 arch/arm64/kernel/traps.c:234
> > [<ffffff9008ca4500>] __dump_stack lib/dump_stack.c:15 [inline]
> > [<ffffff9008ca4500>] dump_stack+0x1a8/0x230 lib/dump_stack.c:51
> > [<ffffff9008d7e078>] ubsan_epilogue+0x34/0x9c lib/ubsan.c:164
> > [<ffffff9008d7ebb4>] handle_overflow+0x228/0x280 lib/ubsan.c:195
> > [<ffffff9008d7ed28>] __ubsan_handle_add_overflow+0x4c/0x68 lib/ubsan.c:203
> > [<ffffff900874c2b8>] SYSC_sync_file_range fs/sync.c:298 [inline]
> > [<ffffff900874c2b8>] SyS_sync_file_range+0x350/0x3e8 fs/sync.c:285
> > [<ffffff9008094480>] el0_svc_naked+0x30/0x34
> > 
> > When calculate the endbyte, there maybe an overflow, even if no effect
> > the kernel, but I also want to avoid overflowing and avoid UBSAN reporting.
> > The original compare is to ensure the offset >= 0 && nbytes >= 0 && no
> > overflow happened.
> > 
> > I do the calculate after compare. ensure the offset >= 0 && nbytes >= 0 &&
> > no overflow may happen first.
> > 
> > Signed-off-by: SunKe <sunke32@xxxxxxxxxx>

I don't have the original mail in my mailbox to reply, let me qote the
code here again:

@@ -246,15 +246,15 @@ int sync_file_range(struct file *file, loff_t offset, loff_t nbytes,
 	if (flags & ~VALID_FLAGS)
 		goto out;
 
-	endbyte = offset + nbytes;
-
 	if ((s64)offset < 0)
 		goto out;
-	if ((s64)endbyte < 0)
+	if ((s64)nbytes < 0)
 		goto out;
-	if (endbyte < offset)
+	if (S64_MAX - offset < nbytes)
 		goto out;
 
+	endbyte = offset + nbytes;

Can this be replaced by check_add_overflow? This can handle
signed/unsigned types while the opencoding obscures the meaning.

And a shameless plug, I sent a fix for another UB report, in remap_verify_area
https://lore.kernel.org/lkml/20190808123942.19592-1-dsterba@xxxxxxxx/

that I'd like to get merged.