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SubjectRe: [PATCH v5 2/6] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount property
On Tue, May 05, 2020 at 05:31:52PM +0200, Mickaël Salaün wrote:
> This new MAY_EXECMOUNT flag enables to check if the underlying mount
> point of an inode is marked as executable. This is useful to implement
> a security policy taking advantage of the noexec mount option.
>
> This flag is set according to path_noexec(), which checks if a mount
> point is mounted with MNT_NOEXEC or if the underlying superblock is
> SB_I_NOEXEC.
>
> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> Reviewed-by: Philippe Trébuchet <philippe.trebuchet@ssi.gouv.fr>
> Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
> Cc: Aleksa Sarai <cyphar@cyphar.com>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Kees Cook <keescook@chromium.org>
> ---
> fs/namei.c | 2 ++
> include/linux/fs.h | 2 ++
> 2 files changed, 4 insertions(+)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index a320371899cf..33b6d372e74a 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2849,6 +2849,8 @@ static int may_open(const struct path *path, int acc_mode, int flag)
> break;
> }
>
> + /* Pass the mount point executability. */
> + acc_mode |= path_noexec(path) ? 0 : MAY_EXECMOUNT;
> error = inode_permission(inode, MAY_OPEN | acc_mode);
> if (error)
> return error;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 313c934de9ee..79435fca6c3e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -103,6 +103,8 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
> #define MAY_NOT_BLOCK 0x00000080
> /* the inode is opened with O_MAYEXEC */
> #define MAY_OPENEXEC 0x00000100
> +/* the mount point is marked as executable */
> +#define MAY_EXECMOUNT 0x00000200
>
> /*
> * flags in file.f_mode. Note that FMODE_READ and FMODE_WRITE must correspond

I find this name unintuitive, but I cannot think of anything better,
since I think my problem is that "MAY" doesn't map to the language I
want to use to describe what this flag is indicating.

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook

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