On Tue, 2020-09-08 at 14:43 +0200, Mickaël Salaün wrote:
> On 08/09/2020 14:28, Mimi Zohar wrote:
> > Hi Mickael,
> >
> > On Tue, 2020-09-08 at 09:59 +0200, Mickaël Salaün wrote:
> >> 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
>
> >
> >> + 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;
