On Sat, Oct 19, 2019 at 03:50:57PM +1100, Aleksa Sarai wrote:
> 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.
Shrug, I wanted to make this consistent with O_NOATIME, which uses
EPERM. EACCES seems more appropriate for lacking permissions for a
particular path rather than for an operation, but the lines are blurry.
> > +
> > /* 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.
This is where we check permissions for O_NOATIME, shouldn't we keep all
of those permission checks in the same place? build_open_flags() only
checks for flag validity.
> > +
> > 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.)
Good catch, I'll fix that. Thanks!
