mm/memfd: add F_SEAL_EXEC

Patch series "mm/memfd: introduce MFD_NOEXEC_SEAL and MFD_EXEC", v8.

Since Linux introduced the memfd feature, memfd have always had their
execute bit set, and the memfd_create() syscall doesn't allow setting it
differently.

However, in a secure by default system, such as ChromeOS, (where all
executables should come from the rootfs, which is protected by Verified
boot), this executable nature of memfd opens a door for NoExec bypass and
enables “confused deputy attack”.  E.g, in VRP bug [1]: cros_vm
process created a memfd to share the content with an external process,
however the memfd is overwritten and used for executing arbitrary code and
root escalation.  [2] lists more VRP in this kind.

On the other hand, executable memfd has its legit use, runc uses memfd’s
seal and executable feature to copy the contents of the binary then
execute them, for such system, we need a solution to differentiate runc's
use of executable memfds and an attacker's [3].

To address those above, this set of patches add following:
1> Let memfd_create() set X bit at creation time.
2> Let memfd to be sealed for modifying X bit.
3> A new pid namespace sysctl: vm.memfd_noexec to control the behavior of
   X bit.For example, if a container has vm.memfd_noexec=2, then
   memfd_create() without MFD_NOEXEC_SEAL will be rejected.
4> A new security hook in memfd_create(). This make it possible to a new
   LSM, which rejects or allows executable memfd based on its security policy.

This patch (of 5):

The new F_SEAL_EXEC flag will prevent modification of the exec bits:
written as traditional octal mask, 0111, or as named flags, S_IXUSR |
S_IXGRP | S_IXOTH.  Any chmod(2) or similar call that attempts to modify
any of these bits after the seal is applied will fail with errno EPERM.

This will preserve the execute bits as they are at the time of sealing, so
the memfd will become either permanently executable or permanently
un-executable.

Link: https://lkml.kernel.org/r/20221215001205.51969-1-jeffxu@google.com
Link: https://lkml.kernel.org/r/20221215001205.51969-2-jeffxu@google.com
Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Co-developed-by: Jeff Xu <jeffxu@google.com>
Signed-off-by: Jeff Xu <jeffxu@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jorge Lucangeli Obes <jorgelo@chromium.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: kernel test robot <lkp@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 2f86b2ad6d7e9d6bd7478c369dc301899a37a791..e8c07da58c9f20ed3b7b4aa886f964d335fa41d2 100644 (file)
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -43,6 +43,7 @@
 #define F_SEAL_GROW    0x0004  /* prevent file from growing */
 #define F_SEAL_WRITE   0x0008  /* prevent writes */
 #define F_SEAL_FUTURE_WRITE    0x0010  /* prevent future writes while mapped */
+#define F_SEAL_EXEC    0x0020  /* prevent chmod modifying exec bits */
 /* (1U << 31) is reserved for signed error codes */
 
 /*

diff --git a/mm/memfd.c b/mm/memfd.c
index 08f5f8304746fb675f744ed93eaa7eaf7e53a6d6..4ebeab94aa74f010519e3184c4695d9a5ac55378 100644 (file)
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -147,6 +147,7 @@ static unsigned int *memfd_file_seals_ptr(struct file *file)
 }
 
 #define F_ALL_SEALS (F_SEAL_SEAL | \
+                    F_SEAL_EXEC | \
                     F_SEAL_SHRINK | \
                     F_SEAL_GROW | \
                     F_SEAL_WRITE | \
@@ -175,6 +176,7 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
         *   SEAL_SHRINK: Prevent the file from shrinking
         *   SEAL_GROW: Prevent the file from growing
         *   SEAL_WRITE: Prevent write access to the file
+        *   SEAL_EXEC: Prevent modification of the exec bits in the file mode
         *
         * As we don't require any trust relationship between two parties, we
         * must prevent seals from being removed. Therefore, sealing a file

diff --git a/mm/shmem.c b/mm/shmem.c
index 0005ab2c29af7bc3241bd50514036c2cdbf7a82b..d3f0c94f68364cac33db3115a33468863a29a421 100644 (file)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1093,6 +1093,12 @@ static int shmem_setattr(struct user_namespace *mnt_userns,
        if (error)
                return error;
 
+       if ((info->seals & F_SEAL_EXEC) && (attr->ia_valid & ATTR_MODE)) {
+               if ((inode->i_mode ^ attr->ia_mode) & 0111) {
+                       return -EPERM;
+               }
+       }
+
        if (S_ISREG(inode->i_mode) && (attr->ia_valid & ATTR_SIZE)) {
                loff_t oldsize = inode->i_size;
                loff_t newsize = attr->ia_size;