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commit 594cc251fdd0d231d342d88b2fdff4bc42fb0690
parent 0b2c8f8b6b0c7530e2866c95862546d0da2057b0
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri,  4 Jan 2019 12:56:09 -0800

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.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Diffstat:
March/x86/include/asm/uaccess.h | 9++++++++-
Mdrivers/gpu/drm/i915/i915_gem_execbuffer.c | 15+++++++++++++--
Minclude/linux/uaccess.h | 2+-
Mkernel/compat.c | 6++----
Mkernel/exit.c | 6++----
Mlib/strncpy_from_user.c | 9+++++----
Mlib/strnlen_user.c | 9+++++----
7 files changed, 36 insertions(+), 20 deletions(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h @@ -705,7 +705,14 @@ extern struct movsl_mask { * checking before using them, but you have to surround them with the * user_access_begin/end() pair. */ -#define user_access_begin() __uaccess_begin() +static __must_check inline bool user_access_begin(const void __user *ptr, size_t len) +{ + if (unlikely(!access_ok(ptr,len))) + return 0; + __uaccess_begin(); + return 1; +} +#define user_access_begin(a,b) user_access_begin(a,b) #define user_access_end() __uaccess_end() #define unsafe_put_user(x, ptr, err_label) \ diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c @@ -1624,7 +1624,9 @@ end_user: * happened we would make the mistake of assuming that the * relocations were valid. */ - user_access_begin(); + if (!user_access_begin(urelocs, size)) + goto end_user; + for (copied = 0; copied < nreloc; copied++) unsafe_put_user(-1, &urelocs[copied].presumed_offset, @@ -2606,7 +2608,16 @@ i915_gem_execbuffer2_ioctl(struct drm_device *dev, void *data, unsigned int i; /* Copy the new buffer offsets back to the user's exec list. */ - user_access_begin(); + /* + * Note: count * sizeof(*user_exec_list) does not overflow, + * because we checked 'count' in check_buffer_count(). + * + * And this range already got effectively checked earlier + * when we did the "copy_from_user()" above. + */ + if (!user_access_begin(user_exec_list, count * sizeof(*user_exec_list))) + goto end_user; + for (i = 0; i < args->buffer_count; i++) { if (!(exec2_list[i].offset & UPDATE)) continue; diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h @@ -264,7 +264,7 @@ extern long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count); probe_kernel_read(&retval, addr, sizeof(retval)) #ifndef user_access_begin -#define user_access_begin() do { } while (0) +#define user_access_begin(ptr,len) access_ok(ptr, len) #define user_access_end() do { } while (0) #define unsafe_get_user(x, ptr, err) do { if (unlikely(__get_user(x, ptr))) goto err; } while (0) #define unsafe_put_user(x, ptr, err) do { if (unlikely(__put_user(x, ptr))) goto err; } while (0) diff --git a/kernel/compat.c b/kernel/compat.c @@ -354,10 +354,9 @@ long compat_get_bitmap(unsigned long *mask, const compat_ulong_t __user *umask, bitmap_size = ALIGN(bitmap_size, BITS_PER_COMPAT_LONG); nr_compat_longs = BITS_TO_COMPAT_LONGS(bitmap_size); - if (!access_ok(umask, bitmap_size / 8)) + if (!user_access_begin(umask, bitmap_size / 8)) return -EFAULT; - user_access_begin(); while (nr_compat_longs > 1) { compat_ulong_t l1, l2; unsafe_get_user(l1, umask++, Efault); @@ -384,10 +383,9 @@ long compat_put_bitmap(compat_ulong_t __user *umask, unsigned long *mask, bitmap_size = ALIGN(bitmap_size, BITS_PER_COMPAT_LONG); nr_compat_longs = BITS_TO_COMPAT_LONGS(bitmap_size); - if (!access_ok(umask, bitmap_size / 8)) + if (!user_access_begin(umask, bitmap_size / 8)) return -EFAULT; - user_access_begin(); while (nr_compat_longs > 1) { unsigned long m = *mask++; unsafe_put_user((compat_ulong_t)m, umask++, Efault); diff --git a/kernel/exit.c b/kernel/exit.c @@ -1604,10 +1604,9 @@ SYSCALL_DEFINE5(waitid, int, which, pid_t, upid, struct siginfo __user *, if (!infop) return err; - if (!access_ok(infop, sizeof(*infop))) + if (!user_access_begin(infop, sizeof(*infop))) return -EFAULT; - user_access_begin(); unsafe_put_user(signo, &infop->si_signo, Efault); unsafe_put_user(0, &infop->si_errno, Efault); unsafe_put_user(info.cause, &infop->si_code, Efault); @@ -1732,10 +1731,9 @@ COMPAT_SYSCALL_DEFINE5(waitid, if (!infop) return err; - if (!access_ok(infop, sizeof(*infop))) + if (!user_access_begin(infop, sizeof(*infop))) return -EFAULT; - user_access_begin(); unsafe_put_user(signo, &infop->si_signo, Efault); unsafe_put_user(0, &infop->si_errno, Efault); unsafe_put_user(info.cause, &infop->si_code, Efault); diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c @@ -114,10 +114,11 @@ long strncpy_from_user(char *dst, const char __user *src, long count) kasan_check_write(dst, count); check_object_size(dst, count, false); - user_access_begin(); - retval = do_strncpy_from_user(dst, src, count, max); - user_access_end(); - return retval; + if (user_access_begin(src, max)) { + retval = do_strncpy_from_user(dst, src, count, max); + user_access_end(); + return retval; + } } return -EFAULT; } diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c @@ -114,10 +114,11 @@ long strnlen_user(const char __user *str, long count) unsigned long max = max_addr - src_addr; long retval; - user_access_begin(); - retval = do_strnlen_user(str, count, max); - user_access_end(); - return retval; + if (user_access_begin(str, max)) { + retval = do_strnlen_user(str, count, max); + user_access_end(); + return retval; + } } return 0; }