[PATCH v6 1/6] mm/memfd: add F_SEAL_EXEC

[jeffxu@chromium.org]

From: Daniel Verkamp <dverkamp@chromium.org>

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.

Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Co-developed-by: Jeff Xu <jeffxu@google.com>
Signed-off-by: Jeff Xu <jeffxu@google.com>
---
 include/uapi/linux/fcntl.h | 1 +
 mm/memfd.c                 | 2 ++
 mm/shmem.c                 | 6 ++++++
 3 files changed, 9 insertions(+)

diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 2f86b2ad6d7e..e8c07da58c9f 100644
--- 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 08f5f8304746..4ebeab94aa74 100644
--- 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 c1d8b8a1aa3b..e18a9cf9d937 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1085,6 +1085,12 @@ 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;
-- 
2.39.0.rc0.267.gcb52ba06e7-goog