Re: [RFC PATCH v8 1/3] fs: Introduce AT_INTERPRETED flag for faccessat2(2)
From: Mimi Zohar <zohar@linux.ibm.com>
Date: 2020-09-08 19:41:51
Also in:
linux-api, linux-fsdevel, linux-security-module, lkml
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.
+ 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
+ }
+ } 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.