On 2019-10-15, Omar Sandoval <osandov@xxxxxxxxxxx> wrote:
> From: Omar Sandoval <osandov@xxxxxx>
>
> The upcoming RWF_ENCODED operation introduces some security concerns:
>
> 1. Compressed writes will pass arbitrary data to decompression
> algorithms in the kernel.
> 2. Compressed reads can leak truncated/hole punched data.
>
> Therefore, we need to require privilege for RWF_ENCODED. It's not
> possible to do the permissions checks at the time of the read or write
> because, e.g., io_uring submits IO from a worker thread. So, add an open
> flag which requires CAP_SYS_ADMIN. It can also be set and cleared with
> fcntl(). The flag is not cleared in any way on fork or exec; it should
> probably be used with O_CLOEXEC in most cases.
>
> Note that the usual issue that unknown open flags are ignored doesn't
> really matter for O_ENCODED; if the kernel doesn't support O_ENCODED,
> then it doesn't support RWF_ENCODED, either.
>
> Signed-off-by: Omar Sandoval <osandov@xxxxxx>
> ---
> fs/fcntl.c | 10 ++++++++--
> fs/namei.c | 4 ++++
> include/linux/fcntl.h | 2 +-
> include/uapi/asm-generic/fcntl.h | 4 ++++
> 4 files changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/fs/fcntl.c b/fs/fcntl.c
> index 3d40771e8e7c..45ebc6df078e 100644
> --- a/fs/fcntl.c
> +++ b/fs/fcntl.c
> @@ -30,7 +30,8 @@
> #include <asm/siginfo.h>
> #include <linux/uaccess.h>
>
> -#define SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | O_DIRECT | O_NOATIME)
> +#define SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | O_DIRECT | O_NOATIME | \
> + O_ENCODED)
>
> static int setfl(int fd, struct file * filp, unsigned long arg)
> {
> @@ -49,6 +50,11 @@ static int setfl(int fd, struct file * filp, unsigned long arg)
> if (!inode_owner_or_capable(inode))
> return -EPERM;
>
> + /* O_ENCODED can only be set by superuser */
> + if ((arg & O_ENCODED) && !(filp->f_flags & O_ENCODED) &&
> + !capable(CAP_SYS_ADMIN))
> + return -EPERM;
I have a feeling the error should probably be an EACCES and not EPERM.
> +
> /* required for strict SunOS emulation */
> if (O_NONBLOCK != O_NDELAY)
> if (arg & O_NDELAY)
> @@ -1031,7 +1037,7 @@ static int __init fcntl_init(void)
> * Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY
> * is defined as O_NONBLOCK on some platforms and not on others.
> */
> - BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ !=
> + BUILD_BUG_ON(22 - 1 /* for O_RDONLY being 0 */ !=
> HWEIGHT32(
> (VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) |
> __FMODE_EXEC | __FMODE_NONOTIFY));
> diff --git a/fs/namei.c b/fs/namei.c
> index 671c3c1a3425..ae86b125888a 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2978,6 +2978,10 @@ static int may_open(const struct path *path, int acc_mode, int flag)
> if (flag & O_NOATIME && !inode_owner_or_capable(inode))
> return -EPERM;
>
> + /* O_ENCODED can only be set by superuser */
> + if ((flag & O_ENCODED) && !capable(CAP_SYS_ADMIN))
> + return -EPERM;
I would suggest that this check be put into build_open_flags() rather
than putting it this late in open(). Also, same nit about the error
return as above.
> +
> return 0;
> }
>
> diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
> index d019df946cb2..5fac02479639 100644
> --- a/include/linux/fcntl.h
> +++ b/include/linux/fcntl.h
> @@ -9,7 +9,7 @@
> (O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \
> O_APPEND | O_NDELAY | O_NONBLOCK | O_NDELAY | __O_SYNC | O_DSYNC | \
> FASYNC | O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \
> - O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE)
> + O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE | O_ENCODED)
>
> #ifndef force_o_largefile
> #define force_o_largefile() (!IS_ENABLED(CONFIG_ARCH_32BIT_OFF_T))
> diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
> index 9dc0bf0c5a6e..8c5cbd5942e3 100644
> --- a/include/uapi/asm-generic/fcntl.h
> +++ b/include/uapi/asm-generic/fcntl.h
> @@ -97,6 +97,10 @@
> #define O_NDELAY O_NONBLOCK
> #endif
>
> +#ifndef O_ENCODED
> +#define O_ENCODED 040000000
> +#endif
You should also define this for all of the architectures which don't use
the generic O_* flag values. On alpha, O_PATH is equal to the value you
picked (just be careful on sparc -- 0x4000000 is the next free bit, but
it's used by FMODE_NONOTIFY.)
> +
> #define F_DUPFD 0 /* dup */
> #define F_GETFD 1 /* get close_on_exec */
> #define F_SETFD 2 /* set/clear close_on_exec */
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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