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Subject[PATCH 5.4 071/104] lib: Reduce user_access_begin() boundaries in strncpy_from_user() and strnlen_user()
Date
From: Christophe Leroy <christophe.leroy@c-s.fr>

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@c-s.fr>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
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();

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