[PATCH] fuse: reject a duplicate fd= mount option

[PATCH] fuse: reject a duplicate fd= mount option

[Baokun Li]

fuse_opt_fd() stored the fuse device in ctx->fud and bumped its refcount
unconditionally:

        ctx->fud = fuse_dev_grab(file);

If fd= is given twice (two fsconfig FSCONFIG_SET_FD calls), the second
call overwrites ctx->fud and grabs the new device, while the reference
taken on the first device is never released - a permanent refcount leak
that pins the first fuse_dev until reboot.

Reject a second fd= outright.  ctx is zeroed on allocation, so a non-NULL
ctx->fud reliably means the option was already processed.

Fixes: d42eb23b2ef9 ("fuse: don't require /dev/fuse fd to be kept open during mount")
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 fs/fuse/inode.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 455c7feba057..2372e6ed333f 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -791,6 +791,9 @@ static int fuse_opt_fd(struct fs_context *fsc, struct file *file)
 {
 	struct fuse_fs_context *ctx = fsc->fs_private;
 
+	if (ctx->fud)
+		return invalfc(fsc, "Multiple fd specified");
+
 	if (file->f_op != &fuse_dev_operations)
 		return invalfc(fsc, "fd is not a fuse device");
 	/*
-- 
2.43.7

Re: [PATCH] fuse: reject a duplicate fd= mount option

[Jingbo Xu]

On 8/11/26 12:53 PM, Baokun Li wrote:
> fuse_opt_fd() stored the fuse device in ctx->fud and bumped its refcount
> unconditionally:
> 
>         ctx->fud = fuse_dev_grab(file);
> 
> If fd= is given twice (two fsconfig FSCONFIG_SET_FD calls), the second
> call overwrites ctx->fud and grabs the new device, while the reference
> taken on the first device is never released - a permanent refcount leak
> that pins the first fuse_dev until reboot.
> 
> Reject a second fd= outright.  ctx is zeroed on allocation, so a non-NULL
> ctx->fud reliably means the option was already processed.
> 
> Fixes: d42eb23b2ef9 ("fuse: don't require /dev/fuse fd to be kept open during mount")
> Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>

LGTM.

Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>


> ---
>  fs/fuse/inode.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 455c7feba057..2372e6ed333f 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -791,6 +791,9 @@ static int fuse_opt_fd(struct fs_context *fsc, struct file *file)
>  {
>  	struct fuse_fs_context *ctx = fsc->fs_private;
>  
> +	if (ctx->fud)
> +		return invalfc(fsc, "Multiple fd specified");
> +
>  	if (file->f_op != &fuse_dev_operations)
>  		return invalfc(fsc, "fd is not a fuse device");
>  	/*

-- 
Thanks,
Jingbo