cachestat: fix page cache statistics permission checking

When the 'cachestat()' system call was added in commit cf264e1329
("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 134fca9063 ("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: cf264e1329 ("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>

mm/filemap.c

@@ -4375,6 +4375,20 @@ resched:
 	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;