From: Andrey Konovalov <andreyknvl@google.com>
To: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc: linux-arch <linux-arch@vger.kernel.org>,
Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
Peter Zijlstra <peterz@infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
Aleksa Sarai <cyphar@cyphar.com>, Ingo Molnar <mingo@kernel.org>,
Khalid Aziz <khalid.aziz@oracle.com>,
Christian Brauner <christian.brauner@ubuntu.com>,
linux-snps-arc@lists.infradead.org,
Linus Torvalds <torvalds@linux-foundation.org>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC 2/4] lib/strncpy_from_user: Remove redundant user space pointer range check
Date: Wed, 15 Jan 2020 15:42:42 +0100 [thread overview]
Message-ID: <CAAeHK+zxVw6jOu-NzjR14U_i5cpDynE=OC3D5WswTvqT8o5NhQ@mail.gmail.com> (raw)
In-Reply-To: <20200114200846.29434-3-vgupta@synopsys.com>
On Tue, Jan 14, 2020 at 9:08 PM Vineet Gupta <Vineet.Gupta1@synopsys.com> wrote:
>
> This came up when switching ARC to word-at-a-time interface and using
> generic/optimized strncpy_from_user
>
> It seems the existing code checks for user buffer/string range multiple
> times and one of tem cn be avoided.
>
> There's an open-coded range check which computes @max off of user_addr_max()
> and thus typically way larger than the kernel buffer @count and subsequently
> discarded in do_strncpy_from_user()
>
> if (max > count)
> max = count;
>
> The canonical user_access_begin() => access_ok() follow anyways and even
> with @count it should suffice for an intial range check as is true for
> any copy_{to,from}_user()
>
> And in case actual user space buffer is smaller than kernel dest pointer
> (i.e. @max < @count) the usual string copy, null byte detection would
> abort the process early anyways
>
> Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
> ---
> lib/strncpy_from_user.c | 36 +++++++++++-------------------------
> lib/strnlen_user.c | 28 +++++++---------------------
> 2 files changed, 18 insertions(+), 46 deletions(-)
>
> diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
> index dccb95af6003..a1622d71f037 100644
> --- a/lib/strncpy_from_user.c
> +++ b/lib/strncpy_from_user.c
> @@ -21,22 +21,15 @@
> /*
> * Do a strncpy, return length of string without final '\0'.
> * 'count' is the user-supplied count (return 'count' if we
> - * hit it), 'max' is the address space maximum (and we return
> - * -EFAULT if we hit it).
> + * hit it). If access fails, return -EFAULT.
> */
> static inline long do_strncpy_from_user(char *dst, const char __user *src,
> - unsigned long count, unsigned long max)
> + unsigned long count)
> {
> const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
> + unsigned long max = count;
> 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;
>
> @@ -72,7 +65,7 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src,
> * Uhhuh. We hit 'max'. But was that the user-specified maximum
> * too? If so, that's ok - we got as much as the user asked for.
> */
> - if (res >= count)
> + if (res == count)
> return res;
>
> /*
> @@ -103,25 +96,18 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src,
> */
> long strncpy_from_user(char *dst, const char __user *src, long count)
> {
> - unsigned long max_addr, src_addr;
> -
> if (unlikely(count <= 0))
> return 0;
>
> - max_addr = user_addr_max();
> - src_addr = (unsigned long)untagged_addr(src);
If you end up changing this code, you need to keep the untagged_addr()
logic, otherwise this breaks arm64 tagged address ABI [1].
[1] https://www.kernel.org/doc/html/latest/arm64/tagged-address-abi.html
> - if (likely(src_addr < max_addr)) {
> - unsigned long max = max_addr - src_addr;
> + kasan_check_write(dst, count);
> + check_object_size(dst, count, false);
> + if (user_access_begin(src, count)) {
> long retval;
> -
> - kasan_check_write(dst, count);
> - check_object_size(dst, count, false);
> - if (user_access_begin(src, max)) {
> - retval = do_strncpy_from_user(dst, src, count, max);
> - user_access_end();
> - return retval;
> - }
> + retval = do_strncpy_from_user(dst, src, count);
> + user_access_end();
> + return retval;
> }
> +
> return -EFAULT;
> }
> EXPORT_SYMBOL(strncpy_from_user);
> diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
> index 6c0005d5dd5c..5ce61f303d6e 100644
> --- a/lib/strnlen_user.c
> +++ b/lib/strnlen_user.c
> @@ -20,19 +20,13 @@
> * if it fits in a aligned 'long'. The caller needs to check
> * the return value against "> max".
> */
> -static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
> +static inline long do_strnlen_user(const char __user *src, unsigned long count)
> {
> const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
> unsigned long align, res = 0;
> + unsigned long max = count;
> 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..
> @@ -64,7 +58,7 @@ static inline long do_strnlen_user(const char __user *src, unsigned long count,
> * Uhhuh. We hit 'max'. But was that the user-specified maximum
> * too? If so, return the marker for "too long".
> */
> - if (res >= count)
> + if (res == count)
> return count+1;
>
> /*
> @@ -98,22 +92,14 @@ static inline long do_strnlen_user(const char __user *src, unsigned long count,
> */
> long strnlen_user(const char __user *str, long count)
> {
> - unsigned long max_addr, src_addr;
> -
> if (unlikely(count <= 0))
> return 0;
>
> - max_addr = user_addr_max();
> - src_addr = (unsigned long)untagged_addr(str);
> - if (likely(src_addr < max_addr)) {
> - unsigned long max = max_addr - src_addr;
> + if (user_access_begin(str, count)) {
> long retval;
> -
> - if (user_access_begin(str, max)) {
> - retval = do_strnlen_user(str, count, max);
> - user_access_end();
> - return retval;
> - }
> + retval = do_strnlen_user(str, count);
> + user_access_end();
> + return retval;
> }
> return 0;
> }
> --
> 2.20.1
>
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next prev parent reply other threads:[~2020-01-15 14:43 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-14 20:08 [RFC 0/4] Switching ARC to optimized generic strncpy_from_user Vineet Gupta
2020-01-14 20:08 ` [RFC 1/4] asm-generic/uaccess: don't define inline functions if noinline lib/* in use Vineet Gupta
2020-01-14 20:57 ` Arnd Bergmann
2020-01-15 23:01 ` Vineet Gupta
2020-01-16 11:43 ` Arnd Bergmann
2020-01-14 21:32 ` Linus Torvalds
2020-01-15 9:08 ` Arnd Bergmann
2020-01-15 14:12 ` Al Viro
2020-01-15 14:21 ` Arnd Bergmann
2020-01-14 20:08 ` [RFC 2/4] lib/strncpy_from_user: Remove redundant user space pointer range check Vineet Gupta
2020-01-14 21:22 ` Linus Torvalds
2020-01-14 21:52 ` Vineet Gupta
2020-01-14 23:46 ` Al Viro
2020-01-15 14:42 ` Andrey Konovalov [this message]
2020-01-15 23:00 ` Vineet Gupta
2020-01-14 20:08 ` [RFC 3/4] ARC: uaccess: remove noinline variants of __strncpy_from_user() and friends Vineet Gupta
2020-01-14 20:08 ` [RFC 4/4] ARC: uaccess: use optimized generic __strnlen_user/__strncpy_from_user Vineet Gupta
2020-01-14 20:42 ` Arnd Bergmann
2020-01-14 21:36 ` Vineet Gupta
2020-01-14 21:49 ` Linus Torvalds
2020-01-14 22:14 ` Vineet Gupta
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