An LSM doesn't get path information related to an access request to open
an inode. This new (internal) MAY_EXECMOUNT flag enables an LSM to
check if the underlying mount point of an inode is marked as executable.
This is useful to implement a security policy taking advantage of the
noexec mount option.
This flag is set according to path_noexec(), which checks if a mount
point is mounted with MNT_NOEXEC or if the underlying superblock is
SB_I_NOEXEC.
Signed-off-by: Mickaël Salaün <[email protected]>
Reviewed-by: Philippe Trébuchet <[email protected]>
Reviewed-by: Thibaut Sautereau <[email protected]>
Cc: Aleksa Sarai <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Kees Cook <[email protected]>
---
fs/namei.c | 2 ++
include/linux/fs.h | 2 ++
2 files changed, 4 insertions(+)
diff --git a/fs/namei.c b/fs/namei.c
index a320371899cf..33b6d372e74a 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2849,6 +2849,8 @@ static int may_open(const struct path *path, int acc_mode, int flag)
break;
}
+ /* Pass the mount point executability. */
+ acc_mode |= path_noexec(path) ? 0 : MAY_EXECMOUNT;
error = inode_permission(inode, MAY_OPEN | acc_mode);
if (error)
return error;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index f5be4be7c01d..9213147d8636 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -103,6 +103,8 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
#define MAY_NOT_BLOCK 0x00000080
/* the inode is opened with RESOLVE_MAYEXEC */
#define MAY_OPENEXEC 0x00000100
+/* the mount point is marked as executable */
+#define MAY_EXECMOUNT 0x00000200
/*
* flags in file.f_mode. Note that FMODE_READ and FMODE_WRITE must correspond
--
2.26.2
On Tue, 28 Apr 2020, Micka?l Sala?n wrote:
> An LSM doesn't get path information related to an access request to open
> an inode. This new (internal) MAY_EXECMOUNT flag enables an LSM to
> check if the underlying mount point of an inode is marked as executable.
> This is useful to implement a security policy taking advantage of the
> noexec mount option.
>
> This flag is set according to path_noexec(), which checks if a mount
> point is mounted with MNT_NOEXEC or if the underlying superblock is
> SB_I_NOEXEC.
>
> Signed-off-by: Micka?l Sala?n <[email protected]>
> Reviewed-by: Philippe Tr?buchet <[email protected]>
> Reviewed-by: Thibaut Sautereau <[email protected]>
> Cc: Aleksa Sarai <[email protected]>
> Cc: Al Viro <[email protected]>
> Cc: Kees Cook <[email protected]>
Are there any existing LSMs which plan to use this aspect?
--
James Morris
<[email protected]>
On 01/05/2020 06:02, James Morris wrote:
> On Tue, 28 Apr 2020, Micka?l Sala?n wrote:
>
>> An LSM doesn't get path information related to an access request to open
>> an inode. This new (internal) MAY_EXECMOUNT flag enables an LSM to
>> check if the underlying mount point of an inode is marked as executable.
>> This is useful to implement a security policy taking advantage of the
>> noexec mount option.
>>
>> This flag is set according to path_noexec(), which checks if a mount
>> point is mounted with MNT_NOEXEC or if the underlying superblock is
>> SB_I_NOEXEC.
>>
>> Signed-off-by: Micka?l Sala?n <[email protected]>
>> Reviewed-by: Philippe Tr?buchet <[email protected]>
>> Reviewed-by: Thibaut Sautereau <[email protected]>
>> Cc: Aleksa Sarai <[email protected]>
>> Cc: Al Viro <[email protected]>
>> Cc: Kees Cook <[email protected]>
>
> Are there any existing LSMs which plan to use this aspect?
This commit message was initially for the first version of O_MAYEXEC,
which extended Yama. The current enforcement implementation is now
directly in the FS subsystem (as requested by Kees Cook). However, this
MAY_EXECMOUNT flag is still used by the current FS implementation and it
could still be useful for LSMs.