x86: uaccess: Inhibit speculation past access_ok() in user_access_begin()

Commit 594cc251fdd0 ("make 'user_access_begin()' do 'access_ok()'")
makes the access_ok() check part of the user_access_begin() preceding a
series of 'unsafe' accesses.  This has the desirable effect of ensuring
that all 'unsafe' accesses have been range-checked, without having to
pick through all of the callsites to verify whether the appropriate
checking has been made.

However, the consolidated range check does not inhibit speculation, so
it is still up to the caller to ensure that they are not susceptible to
any speculative side-channel attacks for user addresses that ultimately
fail the access_ok() check.

This is an oversight, so use __uaccess_begin_nospec() to ensure that
speculation is inhibited until the access_ok() check has passed.

Reported-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index a77445d1b0348675b5b750ad6acd617321b49b1b..780f2b42c8efe76b5d78291d4b633dd26b6e5c64 100644 (file)
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -711,7 +711,7 @@ static __must_check inline bool user_access_begin(const void __user *ptr, size_t
 {
        if (unlikely(!access_ok(ptr,len)))
                return 0;
-       __uaccess_begin();
+       __uaccess_begin_nospec();
        return 1;
 }
 #define user_access_begin(a,b) user_access_begin(a,b)