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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

Showing the changes to kernel (2 of 7 files). Show all

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2 files
  1. kernel/compat.c +2 −4
  2. kernel/exit.c +2 −4
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 bitmap_size = ALIGN(bitmap_size, BITS_PER_COMPAT_LONG);354 bitmap_size = ALIGN(bitmap_size, BITS_PER_COMPAT_LONG);
355 nr_compat_longs = BITS_TO_COMPAT_LONGS(bitmap_size);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 return -EFAULT;358 return -EFAULT;
359 359
360 user_access_begin();
361 while (nr_compat_longs > 1) {360 while (nr_compat_longs > 1) {
362 compat_ulong_t l1, l2;361 compat_ulong_t l1, l2;
363 unsafe_get_user(l1, umask++, Efault);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 bitmap_size = ALIGN(bitmap_size, BITS_PER_COMPAT_LONG);383 bitmap_size = ALIGN(bitmap_size, BITS_PER_COMPAT_LONG);
385 nr_compat_longs = BITS_TO_COMPAT_LONGS(bitmap_size);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 return -EFAULT;387 return -EFAULT;
389 388
390 user_access_begin();
391 while (nr_compat_longs > 1) {389 while (nr_compat_longs > 1) {
392 unsigned long m = *mask++;390 unsigned long m = *mask++;
393 unsafe_put_user((compat_ulong_t)m, umask++, Efault);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 if (!infop)1604 if (!infop)
1605 return err;1605 return err;
1606 1606
1607 if (!access_ok(infop, sizeof(*infop)))1607 if (!user_access_begin(infop, sizeof(*infop)))
1608 return -EFAULT;1608 return -EFAULT;
1609 1609
1610 user_access_begin();
1611 unsafe_put_user(signo, &infop->si_signo, Efault);1610 unsafe_put_user(signo, &infop->si_signo, Efault);
1612 unsafe_put_user(0, &infop->si_errno, Efault);1611 unsafe_put_user(0, &infop->si_errno, Efault);
1613 unsafe_put_user(info.cause, &infop->si_code, Efault);1612 unsafe_put_user(info.cause, &infop->si_code, Efault);
@@ -1732,10 +1731,9 @@ COMPAT_SYSCALL_DEFINE5(waitid, · 118 unmodified lines
1732 if (!infop)1731 if (!infop)
1733 return err;1732 return err;
1734 1733
1735 if (!access_ok(infop, sizeof(*infop)))1734 if (!user_access_begin(infop, sizeof(*infop)))
1736 return -EFAULT;1735 return -EFAULT;
1737 1736
1738 user_access_begin();
1739 unsafe_put_user(signo, &infop->si_signo, Efault);1737 unsafe_put_user(signo, &infop->si_signo, Efault);
1740 unsafe_put_user(0, &infop->si_errno, Efault);1738 unsafe_put_user(0, &infop->si_errno, Efault);
1741 unsafe_put_user(info.cause, &infop->si_code, Efault);1739 unsafe_put_user(info.cause, &infop->si_code, Efault);