Hi Mickael,
On Tue, 2020-09-08 at 09:59 +0200, Mickaël Salaün wrote:
quoted hunk ↗ jump to hunk
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.
quoted hunk ↗ jump to hunk
+ 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;
After this change, I'm wondering if it makes sense to add a call to
security_file_permission(). IMA doesn't currently define it, but
could.
Mimi
quoted hunk ↗ jump to hunk
+ }
+ } else if ((mode & MAY_EXEC) && S_ISREG(inode->i_mode)) {
/*
* MAY_EXEC on regular files is denied if the fs is mounted
* with the "noexec" flag.