This is a note to let you know that I've just added the patch titled lib: Reduce user_access_begin() boundaries in strncpy_from_user() and strnlen_user() to the 5.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: lib-reduce-user_access_begin-boundaries-in-strncpy_from_user-and-strnlen_user.patch and it can be found in the queue-5.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From ab10ae1c3bef56c29bac61e1201c752221b87b41 Mon Sep 17 00:00:00 2001 From: Christophe Leroy <christophe.leroy@xxxxxx> Date: Thu, 23 Jan 2020 08:34:18 +0000 Subject: lib: Reduce user_access_begin() boundaries in strncpy_from_user() and strnlen_user() From: Christophe Leroy <christophe.leroy@xxxxxx> commit ab10ae1c3bef56c29bac61e1201c752221b87b41 upstream. The range passed to user_access_begin() by strncpy_from_user() and strnlen_user() starts at 'src' and goes up to the limit of userspace although reads will be limited by the 'count' param. On 32 bits powerpc (book3s/32) access has to be granted for each 256Mbytes segment and the cost increases with the number of segments to unlock. Limit the range with 'count' param. Fixes: 594cc251fdd0 ("make 'user_access_begin()' do 'access_ok()'") Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- lib/strncpy_from_user.c | 14 +++++++------- lib/strnlen_user.c | 14 +++++++------- 2 files changed, 14 insertions(+), 14 deletions(-) --- a/lib/strncpy_from_user.c +++ b/lib/strncpy_from_user.c @@ -30,13 +30,6 @@ static inline long do_strncpy_from_user( const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS; unsigned long res = 0; - /* - * Truncate 'max' to the user-specified limit, so that - * we only have one limit we need to check in the loop - */ - if (max > count) - max = count; - if (IS_UNALIGNED(src, dst)) goto byte_at_a_time; @@ -114,6 +107,13 @@ long strncpy_from_user(char *dst, const unsigned long max = max_addr - src_addr; long retval; + /* + * Truncate 'max' to the user-specified limit, so that + * we only have one limit we need to check in the loop + */ + if (max > count) + max = count; + kasan_check_write(dst, count); check_object_size(dst, count, false); if (user_access_begin(src, max)) { --- a/lib/strnlen_user.c +++ b/lib/strnlen_user.c @@ -27,13 +27,6 @@ static inline long do_strnlen_user(const unsigned long c; /* - * Truncate 'max' to the user-specified limit, so that - * we only have one limit we need to check in the loop - */ - if (max > count) - max = count; - - /* * Do everything aligned. But that means that we * need to also expand the maximum.. */ @@ -109,6 +102,13 @@ long strnlen_user(const char __user *str unsigned long max = max_addr - src_addr; long retval; + /* + * Truncate 'max' to the user-specified limit, so that + * we only have one limit we need to check in the loop + */ + if (max > count) + max = count; + if (user_access_begin(str, max)) { retval = do_strnlen_user(str, count, max); user_access_end(); Patches currently in stable-queue which might be from christophe.leroy@xxxxxx are queue-5.4/readdir-make-user_access_begin-use-the-real-access-range.patch queue-5.4/lib-reduce-user_access_begin-boundaries-in-strncpy_from_user-and-strnlen_user.patch