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Subject[PATCH v2 5/5] doc: Add documentation for the fs.open_mayexec_enforce sysctl
Changes since v1:
* move from LSM/Yama to sysctl/fs

Signed-off-by: Mickaël Salaün <>
Reviewed-by: Philippe Trébuchet <>
Reviewed-by: Thibaut Sautereau <>
Cc: Jonathan Corbet <>
Cc: Kees Cook <>
Cc: Mickaël Salaün <>
Documentation/admin-guide/sysctl/fs.rst | 43 +++++++++++++++++++++++++
1 file changed, 43 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/fs.rst b/Documentation/admin-guide/sysctl/fs.rst
index 2a45119e3331..f2f5bbe428d6 100644
--- a/Documentation/admin-guide/sysctl/fs.rst
+++ b/Documentation/admin-guide/sysctl/fs.rst
@@ -37,6 +37,7 @@ Currently, these files are in /proc/sys/fs:
- inode-nr
- inode-state
- nr_open
+- open_mayexec_enforce
- overflowuid
- overflowgid
- pipe-user-pages-hard
@@ -165,6 +166,48 @@ system needs to prune the inode list instead of allocating

+The ``O_MAYEXEC`` flag can be passed to :manpage:`open(2)` to only open regular
+files that are expected to be executable. If the file is not identified as
+executable, then the syscall returns -EACCES. This may allow a script
+interpreter to check executable permission before reading commands from a file.
+One interesting use case is to enforce a "write xor execute" policy through
+Thanks to this flag, it is possible to enforce the ``noexec`` mount option
+(i.e. the underlying mount point of the file is mounted with MNT_NOEXEC or its
+underlying superblock is SB_I_NOEXEC) not only on ELF binaries but also on
+scripts. This may be possible thanks to script interpreters using the
+``O_MAYEXEC`` flag. The executable permission is then checked before reading
+commands from a file, and thus can enforce the ``noexec`` at the interpreter
+level by propagating this security policy to the scripts. To be fully
+effective, these interpreters also need to handle the other ways to execute
+code (for which the kernel can't help): command line parameters (e.g., option
+``-e`` for Perl), module loading (e.g., option ``-m`` for Python), stdin, file
+sourcing, environment variables, configuration files... According to the
+threat model, it may be acceptable to allow some script interpreters (e.g.
+Bash) to interpret commands from stdin, may it be a TTY or a pipe, because it
+may not be enough to (directly) perform syscalls.
+There is two complementary security policies: enforce the ``noexec`` mount
+option, or enforce executable file permission. These policies are handled by
+the ``fs.open_mayexec_enforce`` sysctl (writable only with ``CAP_MAC_ADMIN``)
+as a bitmask:
+1 - mount restriction:
+ check that the mount options for the underlying VFS mount do not prevent
+ execution.
+2 - file permission restriction:
+ check that the to-be-opened file is marked as executable for the current
+ process (e.g., POSIX permissions).
+Code samples can be found in tools/testing/selftests/exec/omayexec.c and
+ .
overflowgid & overflowuid

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