From 594cc251fdd0d231d342d88b2fdff4bc42fb0690 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Fri, 4 Jan 2019 12:56:09 -0800 Subject: make 'user_access_begin()' do 'access_ok()' Originally, the rule used to be that you'd have to do access_ok() separately, and then user_access_begin() before actually doing the direct (optimized) user access. But experience has shown that people then decide not to do access_ok() at all, and instead rely on it being implied by other operations or similar. Which makes it very hard to verify that the access has actually been range-checked. If you use the unsafe direct user accesses, hardware features (either SMAP - Supervisor Mode Access Protection - on x86, or PAN - Privileged Access Never - on ARM) do force you to use user_access_begin(). But nothing really forces the range check. By putting the range check into user_access_begin(), we actually force people to do the right thing (tm), and the range check vill be visible near the actual accesses. We have way too long a history of people trying to avoid them. Signed-off-by: Linus Torvalds --- lib/strncpy_from_user.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) (limited to 'lib/strncpy_from_user.c') diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c index b53e1b5d80f4..58eacd41526c 100644 --- a/lib/strncpy_from_user.c +++ b/lib/strncpy_from_user.c @@ -114,10 +114,11 @@ long strncpy_from_user(char *dst, const char __user *src, long count) kasan_check_write(dst, count); check_object_size(dst, count, false); - user_access_begin(); - retval = do_strncpy_from_user(dst, src, count, max); - user_access_end(); - return retval; + if (user_access_begin(src, max)) { + retval = do_strncpy_from_user(dst, src, count, max); + user_access_end(); + return retval; + } } return -EFAULT; } -- cgit