Re: [RFC PATCH v8 1/3] fs: Introduce AT_INTERPRETED flag for faccessat2(2)

[Mimi Zohar <zohar@linux.ibm.com>]

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;