Re: [PATCH RESEND v3 1/4] lib: introduce copy_struct_from_user() helper

Kees Cook
2019-09-30T23:37:58Z

On Tue, Oct 01, 2019 at 05:15:23AM +1000, Aleksa Sarai wrote:
> From: Aleksa Sarai <cyphar@cyphar.com>
>
> A common pattern for syscall extensions is increasing the size of a
> struct passed from userspace, such that the zero-value of the new fields
> result in the old kernel behaviour (allowing for a mix of userspace and
> kernel vintages to operate on one another in most cases).
>
> While this interface exists for communication in both directions, only
> one interface is straightforward to have reasonable semantics for
> (userspace passing a struct to the kernel). For kernel returns to
> userspace, what the correct semantics are (whether there should be an
> error if userspace is unaware of a new extension) is very
> syscall-dependent and thus probably cannot be unified between syscalls
> (a good example of this problem is [1]).
>
> Previously there was no common lib/ function that implemented
> the necessary extension-checking semantics (and different syscalls
> implemented them slightly differently or incompletely[2]). Future
> patches replace common uses of this pattern to make use of
> copy_struct_from_user().
>
> Some in-kernel selftests that insure that the handling of alignment and
> various byte patterns are all handled identically to memchr_inv() usage.
>
> [1]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
> robustify sched_read_attr() ABI logic and code")
>
> [2]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
> similar checks to copy_struct_from_user() while rt_sigprocmask(2)
> always rejects differently-sized struct arguments.
>
> Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
> include/linux/bitops.h | 7 +++
> include/linux/uaccess.h | 4 ++
> lib/strnlen_user.c | 8 +--
> lib/test_user_copy.c | 133 ++++++++++++++++++++++++++++++++++++++--
> lib/usercopy.c | 123 +++++++++++++++++++++++++++++++++++++
> 5 files changed, 262 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index cf074bce3eb3..c94a9ff9f082 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -4,6 +4,13 @@
> #include <asm/types.h>
> #include <linux/bits.h>
>
> +/* Set bits in the first 'n' bytes when loaded from memory */
> +#ifdef __LITTLE_ENDIAN
> +# define aligned_byte_mask(n) ((1UL << 8*(n))-1)
> +#else
> +# define aligned_byte_mask(n) (~0xffUL << (BITS_PER_LONG - 8 - 8*(n)))
> +#endif
> +
> #define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE)
> #define BITS_TO_LONGS(nr) DIV_ROUND_UP(nr, BITS_PER_TYPE(long))
>
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 70bbdc38dc37..94f20e6ec6ab 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -231,6 +231,10 @@ __copy_from_user_inatomic_nocache(void *to, const void __user *from,
>
> #endif /* ARCH_HAS_NOCACHE_UACCESS */
>
> +extern int check_zeroed_user(const void __user *from, size_t size);
> +extern int copy_struct_from_user(void *dst, size_t ksize,
> + const void __user *src, size_t usize);
> +
> /*
> * probe_kernel_read(): safely attempt to read from a location
> * @dst: pointer to the buffer that shall take the data
> diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
> index 28ff554a1be8..6c0005d5dd5c 100644
> --- a/lib/strnlen_user.c
> +++ b/lib/strnlen_user.c
> @@ -3,16 +3,10 @@
> #include <linux/export.h>
> #include <linux/uaccess.h>
> #include <linux/mm.h>
> +#include <linux/bitops.h>
>
> #include <asm/word-at-a-time.h>
>
> -/* Set bits in the first 'n' bytes when loaded from memory */
> -#ifdef __LITTLE_ENDIAN
> -# define aligned_byte_mask(n) ((1ul << 8*(n))-1)
> -#else
> -# define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
> -#endif
> -
> /*
> * Do a strnlen, return length of string *with* final '\0'.
> * 'count' is the user-supplied count, while 'max' is the
> diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
> index 67bcd5dfd847..3a17f71029bb 100644
> --- a/lib/test_user_copy.c
> +++ b/lib/test_user_copy.c
> @@ -16,6 +16,7 @@
> #include <linux/slab.h>
> #include <linux/uaccess.h>
> #include <linux/vmalloc.h>
> +#include <linux/random.h>
>
> /*
> * Several 32-bit architectures support 64-bit {get,put}_user() calls.
> @@ -31,14 +32,129 @@
> # define TEST_U64
> #endif
>
> -#define test(condition, msg) \
> -({ \
> - int cond = (condition); \
> - if (cond) \
> - pr_warn("%s\n", msg); \
> - cond; \
> +#define test(condition, msg, ...) \
> +({ \
> + int cond = (condition); \
> + if (cond) \
> + pr_warn("[%d] " msg "\n", __LINE__, ##__VA_ARGS__); \
> + cond; \
> })
>
> +static bool is_zeroed(void *from, size_t size)
> +{
> + return memchr_inv(from, 0x0, size) == NULL;
> +}
> +
> +static int test_check_nonzero_user(char *kmem, char __user *umem, size_t size)
> +{
> + int ret = 0;
> + size_t start, end, i;
> + size_t zero_start = size / 4;
> + size_t zero_end = size - zero_start;
> +
> + /*
> + * We conduct a series of check_nonzero_user() tests on a block of memory
> + * with the following byte-pattern (trying every possible [start,end]
> + * pair):
> + *
> + * [ 00 ff 00 ff ... 00 00 00 00 ... ff 00 ff 00 ]
> + *
> + * And we verify that check_nonzero_user() acts identically to memchr_inv().
> + */
> +
> + memset(kmem, 0x0, size);
> + for (i = 1; i < zero_start; i += 2)
> + kmem[i] = 0xff;
> + for (i = zero_end; i < size; i += 2)
> + kmem[i] = 0xff;
> +
> + ret |= test(copy_to_user(umem, kmem, size),
> + "legitimate copy_to_user failed");
> +
> + for (start = 0; start <= size; start++) {
> + for (end = start; end <= size; end++) {
> + size_t len = end - start;
> + int retval = check_zeroed_user(umem + start, len);
> + int expected = is_zeroed(kmem + start, len);
> +
> + ret |= test(retval != expected,
> + "check_nonzero_user(=%d) != memchr_inv(=%d) mismatch (start=%zu, end=%zu)",
> + retval, expected, start, end);
> + }
> + }
> +
> + return ret;
> +}
> +
> +static int test_copy_struct_from_user(char *kmem, char __user *umem,
> + size_t size)
> +{
> + int ret = 0;
> + char *rand = NULL, *expected = NULL;
> + size_t ksize, usize;
> +
> + rand = kmalloc(size, GFP_KERNEL);
> + if (ret |= test(rand == NULL, "kmalloc failed"))
> + goto out_free;
> +
> + expected = kmalloc(size, GFP_KERNEL);
> + if (ret |= test(expected == NULL, "kmalloc failed"))
> + goto out_free;
> +
> + /* Fill umem with random bytes. */
> + memset(kmem, 0x0, size);
> + prandom_bytes(rand, size);

I don't really like using random() in tests on the chance that we get
failures we can't reproduced. If you want to do this (instead of using a
byte-fill pattern), you need to dump the entire state of the memory
region. Why not just memset(rand, 0xff, ...)? (And obviously rename
"rand")

> + ret |= test(copy_to_user(umem, rand, size),
> + "legitimate copy_to_user failed");
> +
> + /* Check basic case -- (usize == ksize). */
> + ksize = size;
> + usize = size;

I'd move the memset(kmem, 0x0, size); down to here.

> + memcpy(expected, rand, ksize);
> +
> + ret |= test(copy_struct_from_user(kmem, ksize, umem, usize),
> + "copy_struct_from_user(usize == ksize) failed");
> + ret |= test(memcmp(kmem, expected, ksize),
> + "copy_struct_from_user(usize == ksize) gives unexpected copy");
> +
> + /* Old userspace case -- (usize < ksize). */
> + ksize = size;
> + usize = ksize / 2;
> +

I would expect memset(kmem, 0x0, size); again here since a new test of
that region is starting.

> + memcpy(expected, rand, usize);
> + memset(expected + usize, 0x0, ksize - usize);
> +
> + ret |= test(copy_struct_from_user(kmem, ksize, umem, usize),
> + "copy_struct_from_user(usize < ksize) failed");
> + ret |= test(memcmp(kmem, expected, ksize),
> + "copy_struct_from_user(usize < ksize) gives unexpected copy");
> +
> + /* New userspace (-E2BIG) case -- (usize > ksize). */
> + usize = size;
> + ksize = usize / 2;

and here?

> +
> + ret |= test(copy_struct_from_user(kmem, ksize, umem, usize) != -E2BIG,
> + "copy_struct_from_user(usize > ksize) didn't give E2BIG");
> +
> + /* New userspace (success) case -- (usize > ksize). */
> + usize = size;
> + ksize = usize / 2;
> +

and here?

> + memcpy(expected, rand, ksize);
> +
> + ret |= test(clear_user(umem + ksize, usize - ksize),
> + "legitimate clear_user failed");
> + ret |= test(copy_struct_from_user(kmem, ksize, umem, usize),
> + "copy_struct_from_user(usize > ksize) failed");
> + ret |= test(memcmp(kmem, expected, ksize),
> + "copy_struct_from_user(usize > ksize) gives unexpected copy");
> +
> +out_free:
> + kfree(expected);
> + kfree(rand);
> + return ret;
> +}
> +
> static int __init test_user_copy_init(void)
> {
> int ret = 0;
> @@ -106,6 +222,11 @@ static int __init test_user_copy_init(void)
> #endif
> #undef test_legit
>
> + /* Test usage of check_nonzero_user(). */
> + ret |= test_check_nonzero_user(kmem, usermem, 2 * PAGE_SIZE);
> + /* Test usage of copy_struct_from_user(). */
> + ret |= test_copy_struct_from_user(kmem, usermem, 2 * PAGE_SIZE);
> +
> /*
> * Invalid usage: none of these copies should succeed.
> */
> diff --git a/lib/usercopy.c b/lib/usercopy.c
> index c2bfbcaeb3dc..cf7f854ed9c8 100644
> --- a/lib/usercopy.c
> +++ b/lib/usercopy.c
> @@ -1,5 +1,6 @@
> // SPDX-License-Identifier: GPL-2.0
> #include <linux/uaccess.h>
> +#include <linux/bitops.h>
>
> /* out-of-line parts */
>
> @@ -31,3 +32,125 @@ unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
> }
> EXPORT_SYMBOL(_copy_to_user);
> #endif
> +
> +/**
> + * check_zeroed_user: check if a userspace buffer only contains zero bytes
> + * @from: Source address, in userspace.
> + * @size: Size of buffer.
> + *
> + * This is effectively shorthand for "memchr_inv(from, 0, size) == NULL" for
> + * userspace addresses (and is more efficient because we don't care where the
> + * first non-zero byte is).
> + *
> + * Returns:
> + * * 0: There were non-zero bytes present in the buffer.
> + * * 1: The buffer was full of zero bytes.
> + * * -EFAULT: access to userspace failed.
> + */
> +int check_zeroed_user(const void __user *from, size_t size)
> +{
> + unsigned long val;
> + uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
> +
> + if (unlikely(size == 0))
> + return 1;
> +
> + from -= align;
> + size += align;
> +
> + if (!user_access_begin(from, size))
> + return -EFAULT;
> +
> + unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> + if (align)
> + val &= ~aligned_byte_mask(align);
> +
> + while (size > sizeof(unsigned long)) {
> + if (unlikely(val))
> + goto done;
> +
> + from += sizeof(unsigned long);
> + size -= sizeof(unsigned long);
> +
> + unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> + }
> +
> + if (size < sizeof(unsigned long))
> + val &= aligned_byte_mask(size);
> +
> +done:
> + user_access_end();
> + return (val == 0);
> +err_fault:
> + user_access_end();
> + return -EFAULT;
> +}
> +EXPORT_SYMBOL(check_zeroed_user);
> +
> +/**
> + * copy_struct_from_user: copy a struct from userspace
> + * @dst: Destination address, in kernel space. This buffer must be @ksize
> + * bytes long.
> + * @ksize: Size of @dst struct.
> + * @src: Source address, in userspace.
> + * @usize: (Alleged) size of @src struct.
> + *
> + * Copies a struct from userspace to kernel space, in a way that guarantees
> + * backwards-compatibility for struct syscall arguments (as long as future
> + * struct extensions are made such that all new fields are *appended* to the
> + * old struct, and zeroed-out new fields have the same meaning as the old
> + * struct).
> + *
> + * @ksize is just sizeof(*dst), and @usize should've been passed by userspace.
> + * The recommended usage is something like the following:
> + *
> + * SYSCALL_DEFINE2(foobar, const struct foo __user *, uarg, size_t, usize)
> + * {
> + * int err;
> + * struct foo karg = {};
> + *
> + * if (usize > PAGE_SIZE)
> + * return -E2BIG;
> + * if (usize < FOO_SIZE_VER0)
> + * return -EINVAL;
> + *
> + * err = copy_struct_from_user(&karg, sizeof(karg), uarg, usize);
> + * if (err)
> + * return err;
> + *
> + * // ...
> + * }
> + *
> + * There are three cases to consider:
> + * * If @usize == @ksize, then it's copied verbatim.
> + * * If @usize < @ksize, then the userspace has passed an old struct to a
> + * newer kernel. The rest of the trailing bytes in @dst (@ksize - @usize)
> + * are to be zero-filled.
> + * * If @usize > @ksize, then the userspace has passed a new struct to an
> + * older kernel. The trailing bytes unknown to the kernel (@usize - @ksize)
> + * are checked to ensure they are zeroed, otherwise -E2BIG is returned.
> + *
> + * Returns (in all cases, some data may have been copied):
> + * * -E2BIG: (@usize > @ksize) and there are non-zero trailing bytes in @src.
> + * * -EFAULT: access to userspace failed.
> + */
> +int copy_struct_from_user(void *dst, size_t ksize,
> + const void __user *src, size_t usize)

I'd like to see this be marked __always_inline so the dst type is known
to the compiler during the copy_from_user() size sanity checks, etc.

> +{
> + size_t size = min(ksize, usize);
> + size_t rest = max(ksize, usize) - size;
> +
> + /* Deal with trailing bytes. */
> + if (usize < ksize) {
> + memset(dst + size, 0, rest);
> + } else if (usize > ksize) {
> + int ret = check_zeroed_user(src + size, rest);
> + if (ret <= 0)
> + return ret ?: -E2BIG;
> + }
> + /* Copy the interoperable parts of the struct. */
> + if (copy_from_user(dst, src, size))
> + return -EFAULT;
> + return 0;
> +}
> +EXPORT_SYMBOL(copy_struct_from_user);
> --
> 2.23.0
>

But besides those things, yes please. :)

--
Kees Cook