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SubjectRe: [PULL] vhost: cleanups and fixes
On Fri, Nov 02, 2018 at 11:46:36AM +0000, Mark Rutland wrote:
> On Thu, Nov 01, 2018 at 04:06:19PM -0700, Linus Torvalds wrote:
> > On Thu, Nov 1, 2018 at 4:00 PM Kees Cook <keescook@chromium.org> wrote:
> > >
> > > + memset(&rsp, 0, sizeof(rsp));
> > > + rsp.response = VIRTIO_SCSI_S_FUNCTION_REJECTED;
> > > + resp = vq->iov[out].iov_base;
> > > + ret = __copy_to_user(resp, &rsp, sizeof(rsp));
> > >
> > > Is it actually safe to trust that iov_base has passed an earlier
> > > access_ok() check here? Why not just use copy_to_user() instead?
> >
> > Good point.
> >
> > We really should have removed those double-underscore things ages ago.
>
> FWIW, on arm64 we always check/sanitize the user address as a result of
> our sanitization of speculated values. Almost all of our uaccess
> routines have an explicit access_ok().
>
> All our uaccess routines mask the user pointer based on addr_limit,
> which prevents speculative or architectural uaccess to kernel addresses
> when addr_limit it USER_DS:
>
> 4d8efc2d5ee4c9cc ("arm64: Use pointer masking to limit uaccess speculation")
>
> We also inhibit speculative stores to addr_limit being forwarded under
> speculation:
>
> c2f0ad4fc089cff8 ("arm64: uaccess: Prevent speculative use of the current addr_limit")
>
> ... and given all that, we folded explicit access_ok() checks into
> __{get,put}_user():
>
> 84624087dd7e3b48 ("arm64: uaccess: Don't bother eliding access_ok checks in __{get, put}_user")
>
> IMO we could/should do the same for __copy_{to,from}_user().
>
> Thanks,
> Mark.

I've tried making access_ok mask the parameter it gets. Works because
access_ok is a macro. Most users pass in a variable so that will block
attempts to use speculation to bypass the access_ok checks.

Not 100% as someone can copy the value before access_ok, but
then it's all mitigation anyway.

Places which call access_ok on a non-lvalue need to be fixed then but
there are not too many of these.

The advantage here is that a code like this:

access_ok
for(...)
__get_user

isn't slowed down as the masking is outside the loop.

OTOH macros changing their arguments are kind of ugly.
What do others think?

Just to show what I mean:

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index aae77eb8491c..c4d12c8f47d7 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -7,6 +7,7 @@
#include <linux/compiler.h>
#include <linux/kasan-checks.h>
#include <linux/string.h>
+#include <linux/nospec.h>
#include <asm/asm.h>
#include <asm/page.h>
#include <asm/smap.h>
@@ -69,6 +70,33 @@ static inline bool __chk_range_not_ok(unsigned long addr, unsigned long size, un
__chk_range_not_ok((unsigned long __force)(addr), size, limit); \
})

+/*
+ * Test whether a block of memory is a valid user space address.
+ * Returns 0 if the range is valid, address itself otherwise.
+ */
+static inline unsigned long __verify_range_nospec(unsigned long addr,
+ unsigned long size,
+ unsigned long limit)
+{
+ /* Be careful about overflow */
+ limit = array_index_nospec(limit, size);
+
+ /*
+ * If we have used "sizeof()" for the size,
+ * we know it won't overflow the limit (but
+ * it might overflow the 'addr', so it's
+ * important to subtract the size from the
+ * limit, not add it to the address).
+ */
+ if (__builtin_constant_p(size)) {
+ return array_index_nospec(addr, limit - size + 1);
+ }
+
+ /* Arbitrary sizes? Be careful about overflow */
+ return array_index_mask_nospec(limit, size) &
+ array_index_nospec(addr, limit - size + 1);
+}
+
#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
# define WARN_ON_IN_IRQ() WARN_ON_ONCE(!in_task())
#else
@@ -95,12 +123,46 @@ static inline bool __chk_range_not_ok(unsigned long addr, unsigned long size, un
* checks that the pointer is in the user space range - after calling
* this function, memory access functions may still return -EFAULT.
*/
-#define access_ok(type, addr, size) \
+#define unsafe_access_ok(type, addr, size) \
({ \
WARN_ON_IN_IRQ(); \
likely(!__range_not_ok(addr, size, user_addr_max())); \
})

+/**
+ * access_ok_nospec: - Checks if a user space pointer is valid
+ * @type: Type of access: %VERIFY_READ or %VERIFY_WRITE. Note that
+ * %VERIFY_WRITE is a superset of %VERIFY_READ - if it is safe
+ * to write to a block, it is always safe to read from it.
+ * @addr: User space pointer to start of block to check
+ * @size: Size of block to check
+ *
+ * Context: User context only. This function may sleep if pagefaults are
+ * enabled.
+ *
+ * Checks if a pointer to a block of memory in user space is valid.
+ *
+ * Returns address itself (nonzero) if the memory block may be valid,
+ * zero if it is definitely invalid.
+ *
+ * To prevent speculation, the returned value must then be used
+ * for accesses.
+ *
+ * Note that, depending on architecture, this function probably just
+ * checks that the pointer is in the user space range - after calling
+ * this function, memory access functions may still return -EFAULT.
+ */
+#define access_ok_nospec(type, addr, size) \
+({ \
+ WARN_ON_IN_IRQ(); \
+ __chk_user_ptr(addr); \
+ addr = (typeof(addr) __force) \
+ __verify_range_nospec((unsigned long __force)(addr), \
+ size, user_addr_max()); \
+})
+
+#define access_ok(type, addr, size) access_ok_nospec(type, addr, size)
+
/*
* These are the main single-value transfer routines. They automatically
* use the right size if we just have the right pointer type.
--
MST
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