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

Unified · Split

2 files

kernel/compat.c +2 −4
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	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);