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

[Jan Kara]

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>

Thanks for the patch. The patch looks good to me. You can add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

Al, care to pickup this fix?

								Honza

> diff --git a/fs/sync.c b/fs/sync.c
> index 4d1ff01..5827471 100644
> --- a/fs/sync.c
> +++ b/fs/sync.c
> @@ -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;
> +
>  	if (sizeof(pgoff_t) == 4) {
>  		if (offset >= (0x100000000ULL << PAGE_SHIFT)) {
>  			/*
> -- 
> 2.7.4
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR