[PATCH v2] usb: gadget: f_mass_storage: fix null pointer dereference in fsg_common_set_num_buffers()

[PATCH v2] usb: gadget: f_mass_storage: fix null pointer dereference in fsg_common_set_num_buffers()

[Jeffin Philip]

Previously fsg_num_buffers_validate() was removed as it was not
necessary due to Kconfig setting the limits for n from 2 to 256 with
default as 2. However, setting the page content in such a way that
kstrtou8() reflects n value as either 0 or 1 bypasses these
restrictions leading to a null pointer dereference if n is 0. Fix
this by adding a check for n < 2 and returning -EINVAL if n is
either 0 or 1 consistent with Kconfig logic.

Reported-by: syzbot+791be35f1fbcc85d06d7@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=791be35f1fbcc85d06d7
Fixes: fe5a6c48fd95 ("usb: gadget: storage: get rid of fsg_num_buffers_validate()")
Cc: stable@vger.kernel.org
Signed-off-by: Jeffin Philip <jeffinphilip14@gmail.com>
---
Changes in v2:
 - Change check from n = 0 to n < 2 consistent with Kconfig logic
---
 drivers/usb/gadget/function/f_mass_storage.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/usb/gadget/function/f_mass_storage.c b/drivers/usb/gadget/function/f_mass_storage.c
index a50743caf083..4e743be220cc 100644
--- a/drivers/usb/gadget/function/f_mass_storage.c
+++ b/drivers/usb/gadget/function/f_mass_storage.c
@@ -2747,6 +2747,9 @@ int fsg_common_set_num_buffers(struct fsg_common *common, unsigned int n)
 	struct fsg_buffhd *bh, *buffhds;
 	int i;
 
+	if (n < 2)
+		return -EINVAL;
+
 	buffhds = kzalloc_objs(*buffhds, n);
 	if (!buffhds)
 		return -ENOMEM;
-- 
2.55.0

Re: [PATCH v2] usb: gadget: f_mass_storage: fix null pointer dereference in fsg_common_set_num_buffers()

[Alan Stern]

On Tue, Aug 18, 2026 at 09:29:04AM +0530, Jeffin Philip wrote:
> Previously fsg_num_buffers_validate() was removed as it was not
> necessary due to Kconfig setting the limits for n from 2 to 256 with
> default as 2. However, setting the page content in such a way that
> kstrtou8() reflects n value as either 0 or 1 bypasses these
> restrictions leading to a null pointer dereference if n is 0. Fix
> this by adding a check for n < 2 and returning -EINVAL if n is
> either 0 or 1 consistent with Kconfig logic.
> 
> Reported-by: syzbot+791be35f1fbcc85d06d7@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=791be35f1fbcc85d06d7
> Fixes: fe5a6c48fd95 ("usb: gadget: storage: get rid of fsg_num_buffers_validate()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Jeffin Philip <jeffinphilip14@gmail.com>

Acked-by: Alan Stern <stern@rowland.harvard.edu>

> ---
> Changes in v2:
>  - Change check from n = 0 to n < 2 consistent with Kconfig logic
> ---
>  drivers/usb/gadget/function/f_mass_storage.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/usb/gadget/function/f_mass_storage.c b/drivers/usb/gadget/function/f_mass_storage.c
> index a50743caf083..4e743be220cc 100644
> --- a/drivers/usb/gadget/function/f_mass_storage.c
> +++ b/drivers/usb/gadget/function/f_mass_storage.c
> @@ -2747,6 +2747,9 @@ int fsg_common_set_num_buffers(struct fsg_common *common, unsigned int n)
>  	struct fsg_buffhd *bh, *buffhds;
>  	int i;
>  
> +	if (n < 2)
> +		return -EINVAL;
> +
>  	buffhds = kzalloc_objs(*buffhds, n);
>  	if (!buffhds)
>  		return -ENOMEM;
> -- 
> 2.55.0
> 
>