[SRU Focal/Bionic] vfs: check fd has read access in kernel_read_file_from_fd()

[Thadeu Lima de Souza Cascardo <cascardo@canonical.com>]

From: "Matthew Wilcox (Oracle)" <willy@infradead.org>

BugLink: https://bugs.launchpad.net/bugs/1950644

commit 032146cda85566abcd1c4884d9d23e4e30a07e9a upstream.

If we open a file without read access and then pass the fd to a syscall
whose implementation calls kernel_read_file_from_fd(), we get a warning
from __kernel_read():

        if (WARN_ON_ONCE(!(file->f_mode & FMODE_READ)))

This currently affects both finit_module() and kexec_file_load(), but it
could affect other syscalls in the future.

Link: https://lkml.kernel.org/r/20211007220110.600005-1-willy@infradead.org
Fixes: b844f0ecbc56 ("vfs: define kernel_copy_file_from_fd()")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reported-by: Hao Sun <sunhao.th@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Mimi Zohar <zohar@linux.ibm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 0f218ba4c8aac7041cd8b81a5a893b0d121e6316 linux-5.4.y)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
---
 fs/exec.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index eeba096e8a38..006f7fb40b96 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1000,7 +1000,7 @@ int kernel_read_file_from_fd(int fd, void **buf, loff_t *size, loff_t max_size,
 	struct fd f = fdget(fd);
 	int ret = -EBADF;
 
-	if (!f.file)
+	if (!f.file || !(f.file->f_mode & FMODE_READ))
 		goto out;
 
 	ret = kernel_read_file(f.file, buf, size, max_size, id);
-- 
2.32.0