From: Omar Sandoval <redacted>

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 must be
combined with O_CLOEXEC when opening to avoid accidental leaks (if
needed, it may be set without O_CLOEXEC by using fnctl()).

Note that the usual issue that unknown open flags are ignored doesn't
really matter for O_ALLOW_ENCODED; if the kernel doesn't support
O_ALLOW_ENCODED, then it doesn't support RWF_ENCODED, either.

Signed-off-by: Omar Sandoval <redacted>
---
 arch/alpha/include/uapi/asm/fcntl.h  |  1 +
 arch/parisc/include/uapi/asm/fcntl.h |  1 +
 arch/sparc/include/uapi/asm/fcntl.h  |  1 +
 fs/fcntl.c                           | 10 ++++++++--
 fs/namei.c                           |  4 ++++
 fs/open.c                            |  7 +++++++
 include/linux/fcntl.h                |  2 +-
 include/uapi/asm-generic/fcntl.h     |  4 ++++
 8 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/arch/alpha/include/uapi/asm/fcntl.h b/arch/alpha/include/uapi/asm/fcntl.h
index 50bdc8e8a271..391e0d112e41 100644
--- a/arch/alpha/include/uapi/asm/fcntl.h
+++ b/arch/alpha/include/uapi/asm/fcntl.h

@@ -34,6 +34,7 @@

 #define O_PATH         040000000
 #define __O_TMPFILE    0100000000
+#define O_ALLOW_ENCODED        0200000000

 #define F_GETLK                7
 #define F_SETLK                8

diff --git a/arch/parisc/include/uapi/asm/fcntl.h b/arch/parisc/include/uapi/asm/fcntl.h
index 03dee816cb13..72ea9bdf5f04 100644
--- a/arch/parisc/include/uapi/asm/fcntl.h
+++ b/arch/parisc/include/uapi/asm/fcntl.h

@@ -19,6 +19,7 @@

 #define O_PATH         020000000
 #define __O_TMPFILE    040000000
+#define O_ALLOW_ENCODED        100000000

 #define F_GETLK64      8
 #define F_SETLK64      9

diff --git a/arch/sparc/include/uapi/asm/fcntl.h b/arch/sparc/include/uapi/asm/fcntl.h
index 67dae75e5274..ac3e8c9cb32c 100644
--- a/arch/sparc/include/uapi/asm/fcntl.h
+++ b/arch/sparc/include/uapi/asm/fcntl.h

@@ -37,6 +37,7 @@

 #define O_PATH         0x1000000
 #define __O_TMPFILE    0x2000000
+#define O_ALLOW_ENCODED        0x8000000

 #define F_GETOWN       5       /*  for sockets. */
 #define F_SETOWN       6       /*  for sockets. */

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 19ac5baad50f..9302f68fe698 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_ALLOW_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_ALLOW_ENCODED can only be set by superuser */
+       if ((arg & O_ALLOW_ENCODED) && !(filp->f_flags & O_ALLOW_ENCODED) &&
+           !capable(CAP_SYS_ADMIN))
+               return -EPERM;
+
        /* required for strict SunOS emulation */
        if (O_NONBLOCK != O_NDELAY)
               if (arg & O_NDELAY)

@@ -1033,7 +1039,7 @@ 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 d4a6dd772303..fbf64ce61088 100644
--- a/fs/namei.c
+++ b/fs/namei.c

@@ -2890,6 +2890,10 @@ 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_ALLOW_ENCODED can only be set by superuser */
+       if ((flag & O_ALLOW_ENCODED) && !capable(CAP_SYS_ADMIN))
+               return -EPERM;
+
        return 0;
 }

diff --git a/fs/open.c b/fs/open.c
index 9af548fb841b..f2863aaf78e7 100644
--- a/fs/open.c
+++ b/fs/open.c

@@ -1040,6 +1040,13 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
                acc_mode = 0;
        }

+       /*
+        * O_ALLOW_ENCODED must be combined with O_CLOEXEC to avoid accidentally
+        * leaking encoded I/O privileges.
+        */
+       if ((how->flags & (O_ALLOW_ENCODED | O_CLOEXEC)) == O_ALLOW_ENCODED)
+               return -EINVAL;
+