cachestat: fix page cache statistics permission checking

JIRA: https://issues.redhat.com/browse/RHEL-78135
JIRA: https://issues.redhat.com/browse/RHEL-78991
CVE: CVE-2025-21691

commit 5f53766
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue Jan 21 09:27:22 2025 -0800

    cachestat: fix page cache statistics permission checking

    When the 'cachestat()' system call was added in commit cf264e1
    ("cachestat: implement cachestat syscall"), it was meant to be a much
    more convenient (and performant) version of mincore() that didn't need
    mapping things into the user virtual address space in order to work.

    But it ended up missing the "check for writability or ownership" fix for
    mincore(), done in commit 134fca9 ("mm/mincore.c: make mincore()
    more conservative").

    This just adds equivalent logic to 'cachestat()', modified for the file
    context (rather than vma).

    Reported-by: Sudheendra Raghav Neela <sneela@tugraz.at>
    Fixes: cf264e1 ("cachestat: implement cachestat syscall")
    Tested-by: Johannes Weiner <hannes@cmpxchg.org>
    Acked-by: Johannes Weiner <hannes@cmpxchg.org>
    Acked-by: Nhat Pham <nphamcs@gmail.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Signed-off-by: Luiz Capitulino <luizcap@redhat.com>

Author: luiz-cap

mm/filemap.c

@@ -4375,6 +4375,20 @@ static void filemap_cachestat(struct address_space *mapping,
 	rcu_read_unlock();
 }
 
+/*
+ * See mincore: reveal pagecache information only for files
+ * that the calling process has write access to, or could (if
+ * tried) open for writing.
+ */
+static inline bool can_do_cachestat(struct file *f)
+{
+	if (f->f_mode & FMODE_WRITE)
+		return true;
+	if (inode_owner_or_capable(file_mnt_idmap(f), file_inode(f)))
+		return true;
+	return file_permission(f, MAY_WRITE) == 0;
+}
+
 /*
  * The cachestat(2) system call.
  *
@@ -4430,6 +4444,9 @@ SYSCALL_DEFINE4(cachestat, unsigned int, fd,
 	if (is_file_hugepages(fd_file(f)))
 		return -EOPNOTSUPP;
 
+	if (!can_do_cachestat(fd_file(f)))
+		return -EPERM;
+
 	if (flags != 0)
 		return -EINVAL;