[PATCH v6 02/11] fs: add O_ALLOW_ENCODED open flag

[Omar Sandoval <osandov@osandov.com>]

From: Omar Sandoval <osandov@fb.com>

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 <osandov@fb.com>
---
 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;
+
 	/*
 	 * O_SYNC is implemented as __O_SYNC|O_DSYNC.  As many places only
 	 * check for O_DSYNC if the need any syncing at all we enforce it's
diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
index 921e750843e6..dc66c557b7d0 100644
--- a/include/linux/fcntl.h
+++ b/include/linux/fcntl.h
@@ -10,7 +10,7 @@
 	(O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \
 	 O_APPEND | O_NDELAY | O_NONBLOCK | __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_ALLOW_ENCODED)
 
 /* List of all valid flags for the how->upgrade_mask argument: */
 #define VALID_UPGRADE_FLAGS \
diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
index 9dc0bf0c5a6e..75321c7a66ac 100644
--- a/include/uapi/asm-generic/fcntl.h
+++ b/include/uapi/asm-generic/fcntl.h
@@ -89,6 +89,10 @@
 #define __O_TMPFILE	020000000
 #endif
 
+#ifndef O_ALLOW_ENCODED
+#define O_ALLOW_ENCODED	040000000
+#endif
+
 /* a horrid kludge trying to make sure that this will fail on old kernels */
 #define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
 #define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT)      
-- 
2.29.2