Re: [PATCH] fs: exfat: fix improper check of dentry.stream.valid_size

[Namjae Jeon]

On Tue, Oct 14, 2025 at 10:02 PM Jaehun Gou <p22gone@xxxxxxxxx> wrote:
>
> We found an infinite loop bug in the exFAT file system that can lead to a
> Denial-of-Service (DoS) condition. When a dentry in an exFAT filesystem is
> malformed, the following system calls — SYS_openat, SYS_ftruncate, and
> SYS_pwrite64 — can cause the kernel to hang.
>
> Root cause analysis shows that the size validation code in exfat_find()
> does not check whether dentry.stream.valid_size is negative. As a result,
> the system calls mentioned above can succeed and eventually trigger the DoS
> issue.
>
> This patch adds a check for negative dentry.stream.valid_size to prevent
> this vulnerability.
>
> Co-developed-by: Seunghun Han <kkamagui@xxxxxxxxx>
> Signed-off-by: Seunghun Han <kkamagui@xxxxxxxxx>
> Co-developed-by: Jihoon Kwon <jimmyxyz010315@xxxxxxxxx>
> Signed-off-by: Jihoon Kwon <jimmyxyz010315@xxxxxxxxx>
> Signed-off-by: Jaehun Gou <p22gone@xxxxxxxxx>
Applied it to #dev.
Thanks!