On Tue, 2020-09-08 at 14:43 +0200, Mickaël Salaün wrote:

On 08/09/2020 14:28, Mimi Zohar wrote:

quoted

Hi Mickael,

On Tue, 2020-09-08 at 09:59 +0200, Mickaël Salaün wrote:

quoted

diff --git a/fs/open.c b/fs/open.c
index 9af548fb841b..879bdfbdc6fa 100644
--- a/fs/open.c
+++ b/fs/open.c

@@ -405,9 +405,13 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla
 	if (mode & ~S_IRWXO)	/* where's F_OK, X_OK, W_OK, R_OK? */
 		return -EINVAL;
 
-	if (flags & ~(AT_EACCESS | AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH))
+	if (flags & ~(AT_EACCESS | AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH |
+				AT_INTERPRETED))
 		return -EINVAL;
 
+	/* Only allows X_OK with AT_INTERPRETED for now. */
+	if ((flags & AT_INTERPRETED) && !(mode & S_IXOTH))
+		return -EINVAL;
 	if (flags & AT_SYMLINK_NOFOLLOW)
 		lookup_flags &= ~LOOKUP_FOLLOW;
 	if (flags & AT_EMPTY_PATH)

@@ -426,7 +430,30 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla
 
 	inode = d_backing_inode(path.dentry);
 
-	if ((mode & MAY_EXEC) && S_ISREG(inode->i_mode)) {
+	if ((flags & AT_INTERPRETED)) {
+		/*
+		 * For compatibility reasons, without a defined security policy
+		 * (via sysctl or LSM), using AT_INTERPRETED must map the
+		 * execute permission to the read permission.  Indeed, from
+		 * user space point of view, being able to execute data (e.g.
+		 * scripts) implies to be able to read this data.
+		 *
+		 * The MAY_INTERPRETED_EXEC bit is set to enable LSMs to add
+		 * custom checks, while being compatible with current policies.
+		 */
+		if ((mode & MAY_EXEC)) {

Why is the ISREG() test being dropped?   Without dropping it, there
would be no reason for making the existing test an "else" clause.

The ISREG() is not dropped, it is just moved below with the rest of the
original code. The corresponding code (with the path_noexec call) for
AT_INTERPRETED is added with the next commit, and it relies on the
sysctl configuration for compatibility reasons.

Dropping the S_ISREG() check here without an explanation is wrong and
probably unsafe, as it is only re-added in the subsequent patch and
only for the "sysctl_interpreted_access" case.  Adding this new test
after the existing test is probably safer.  If the original test fails,
it returns the same value as this test -EACCES.

Mimi

quoted

quoted

+			mode |= MAY_INTERPRETED_EXEC;
+			/*
+			 * For compatibility reasons, if the system-wide policy
+			 * doesn't enforce file permission checks, then
+			 * replaces the execute permission request with a read
+			 * permission request.
+			 */
+			mode &= ~MAY_EXEC;
+			/* To be executed *by* user space, files must be readable. */
+			mode |= MAY_READ;