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.

Linus Torvalds committed 7 years ago

Commit 594cc251 parent 0b2c8f87 files · +36 −20Browse files

Unified · Split

7 files

arch/x86/include/asm/uaccess.h +8 −1
drivers/gpu/drm/i915/i915_gem_execbuffer.c +13 −2
include/linux/uaccess.h +1 −1
kernel/compat.c +2 −4
kernel/exit.c +2 −4
lib/strncpy_from_user.c +5 −4
lib/strnlen_user.c +5 −4

arch/x86/include/asm/uaccess.h+8−1

@@ -705,7 +705,14 @@ extern struct movsl_mask { · 704 unmodified lines
705	705	* checking before using them, but you have to surround them with the
706	706	* user_access_begin/end() pair.
707	707	*/
708		−#define user_access_begin() __uaccess_begin()
	708	+static __must_check inline bool user_access_begin(const void __user *ptr, size_t len)
	709	+{
	710	+ if (unlikely(!access_ok(ptr,len)))
	711	+ return 0;
	712	+ __uaccess_begin();
	713	+ return 1;
	714	+}
	715	+#define user_access_begin(a,b) user_access_begin(a,b)
709	716	#define user_access_end() __uaccess_end()
710	717
711	718	#define unsafe_put_user(x, ptr, err_label) \

drivers/gpu/drm/i915/i915_gem_execbuffer.c+13−2

@@ -1624,7 +1624,9 @@ end_user: · 1623 unmodified lines
1624	1624	* happened we would make the mistake of assuming that the
1625	1625	* relocations were valid.
1626	1626	*/
1627		− user_access_begin();
	1627	+ if (!user_access_begin(urelocs, size))
	1628	+ goto end_user;
	1629	+
1628	1630	for (copied = 0; copied < nreloc; copied++)
1629	1631	unsafe_put_user(-1,
1630	1632	&urelocs[copied].presumed_offset,
@@ -2606,7 +2608,16 @@ i915_gem_execbuffer2_ioctl(struct drm_device dev, void data, · 975 unmodified lines
2606	2608	unsigned int i;
2607	2609
2608	2610	/* Copy the new buffer offsets back to the user's exec list. */
2609		− user_access_begin();
	2611	+ /*
	2612	+ * Note: count * sizeof(*user_exec_list) does not overflow,
	2613	+ * because we checked 'count' in check_buffer_count().
	2614	+ *
	2615	+ * And this range already got effectively checked earlier
	2616	+ * when we did the "copy_from_user()" above.
	2617	+ */
	2618	+ if (!user_access_begin(user_exec_list, count * sizeof(*user_exec_list)))
	2619	+ goto end_user;
	2620	+
2610	2621	for (i = 0; i < args->buffer_count; i++) {
2611	2622	if (!(exec2_list[i].offset & UPDATE))
2612	2623	continue;

include/linux/uaccess.h+1−1

@@ -264,7 +264,7 @@ extern long strncpy_from_unsafe(char dst, const void unsafe_addr, long count); · 263 unmodified lines
264	264	probe_kernel_read(&retval, addr, sizeof(retval))
265	265
266	266	#ifndef user_access_begin
267		−#define user_access_begin() do { } while (0)
	267	+#define user_access_begin(ptr,len) access_ok(ptr, len)
268	268	#define user_access_end() do { } while (0)
269	269	#define unsafe_get_user(x, ptr, err) do { if (unlikely(__get_user(x, ptr))) goto err; } while (0)
270	270	#define unsafe_put_user(x, ptr, err) do { if (unlikely(__put_user(x, ptr))) goto err; } while (0)

kernel/compat.c+2−4

@@ -354,10 +354,9 @@ long compat_get_bitmap(unsigned long mask, const compat_ulong_t __user umask, · 353 unmodified lines
354	354	bitmap_size = ALIGN(bitmap_size, BITS_PER_COMPAT_LONG);
355	355	nr_compat_longs = BITS_TO_COMPAT_LONGS(bitmap_size);
356	356
357		− if (!access_ok(umask, bitmap_size / 8))
	357	+ if (!user_access_begin(umask, bitmap_size / 8))
358	358	return -EFAULT;
359	359
360		− user_access_begin();
361	360	while (nr_compat_longs > 1) {
362	361	compat_ulong_t l1, l2;
363	362	unsafe_get_user(l1, umask++, Efault);
@@ -384,10 +383,9 @@ long compat_put_bitmap(compat_ulong_t __user umask, unsigned long mask, · 20 unmodified lines
384	383	bitmap_size = ALIGN(bitmap_size, BITS_PER_COMPAT_LONG);
385	384	nr_compat_longs = BITS_TO_COMPAT_LONGS(bitmap_size);
386	385
387		− if (!access_ok(umask, bitmap_size / 8))
	386	+ if (!user_access_begin(umask, bitmap_size / 8))
388	387	return -EFAULT;
389	388
390		− user_access_begin();
391	389	while (nr_compat_longs > 1) {
392	390	unsigned long m = *mask++;
393	391	unsafe_put_user((compat_ulong_t)m, umask++, Efault);

kernel/exit.c+2−4

@@ -1604,10 +1604,9 @@ SYSCALL_DEFINE5(waitid, int, which, pid_t, upid, struct siginfo __user *, · 1603 unmodified lines
1604	1604	if (!infop)
1605	1605	return err;
1606	1606
1607		− if (!access_ok(infop, sizeof(*infop)))
	1607	+ if (!user_access_begin(infop, sizeof(*infop)))
1608	1608	return -EFAULT;
1609	1609
1610		− user_access_begin();
1611	1610	unsafe_put_user(signo, &infop->si_signo, Efault);
1612	1611	unsafe_put_user(0, &infop->si_errno, Efault);
1613	1612	unsafe_put_user(info.cause, &infop->si_code, Efault);
@@ -1732,10 +1731,9 @@ COMPAT_SYSCALL_DEFINE5(waitid, · 118 unmodified lines
1732	1731	if (!infop)
1733	1732	return err;
1734	1733
1735		− if (!access_ok(infop, sizeof(*infop)))
	1734	+ if (!user_access_begin(infop, sizeof(*infop)))
1736	1735	return -EFAULT;
1737	1736
1738		− user_access_begin();
1739	1737	unsafe_put_user(signo, &infop->si_signo, Efault);
1740	1738	unsafe_put_user(0, &infop->si_errno, Efault);
1741	1739	unsafe_put_user(info.cause, &infop->si_code, Efault);

lib/strncpy_from_user.c+5−4

@@ -114,10 +114,11 @@ long strncpy_from_user(char dst, const char __user src, long count) · 113 unmodified lines
114	114
115	115	kasan_check_write(dst, count);
116	116	check_object_size(dst, count, false);
117		− user_access_begin();
118		− retval = do_strncpy_from_user(dst, src, count, max);
119		− user_access_end();
120		− return retval;
	117	+ if (user_access_begin(src, max)) {
	118	+ retval = do_strncpy_from_user(dst, src, count, max);
	119	+ user_access_end();
	120	+ return retval;
	121	+ }
121	122	}
122	123	return -EFAULT;
123	124	}

lib/strnlen_user.c+5−4

@@ -114,10 +114,11 @@ long strnlen_user(const char __user *str, long count) · 113 unmodified lines
114	114	unsigned long max = max_addr - src_addr;
115	115	long retval;
116	116
117		− user_access_begin();
118		− retval = do_strnlen_user(str, count, max);
119		− user_access_end();
120		− return retval;
	117	+ if (user_access_begin(str, max)) {
	118	+ retval = do_strnlen_user(str, count, max);
	119	+ user_access_end();
	120	+ return retval;
	121	+ }
121	122	}
122	123	return 0;
123	124	}