powerpc/32s: Fix kuap_kernel_restore()

At interrupt exit, kuap_kernel_restore() calls kuap_unlock() with the
value contained in regs->kuap. However, when regs->kuap contains
0xffffffff it means that KUAP was not unlocked so calling kuap_unlock()
is unrelevant and results in jeopardising the contents of kernel space
segment registers.

So check that regs->kuap doesn't contain KUAP_NONE before calling
kuap_unlock(). In the meantime it also means that if KUAP has not
been correcly locked back at interrupt exit, it must be locked
before continuing. This is done by checking the content of
current->thread.kuap which was returned by kuap_get_and_assert_locked()

Fixes: 6f1f1e509039 ("powerpc/32s: Rework Kernel Userspace Access Protection")
Reported-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/0d0c4d0f050a637052287c09ba521bad960a2790.1631715131.git.christophe.leroy@csgroup.eu

diff --git a/arch/powerpc/include/asm/book3s/32/kup.h b/arch/powerpc/include/asm/book3s/32/kup.h
index d4b145b279f6c51bc2d644720af92db03e488350..9f38040f0641dc1b87e036f77b961961b21eab80 100644 (file)
--- a/arch/powerpc/include/asm/book3s/32/kup.h
+++ b/arch/powerpc/include/asm/book3s/32/kup.h
@@ -136,6 +136,14 @@ static inline void kuap_kernel_restore(struct pt_regs *regs, unsigned long kuap)
        if (kuap_is_disabled())
                return;
 
+       if (unlikely(kuap != KUAP_NONE)) {
+               current->thread.kuap = KUAP_NONE;
+               kuap_lock(kuap, false);
+       }
+
+       if (likely(regs->kuap == KUAP_NONE))
+               return;
+
        current->thread.kuap = regs->kuap;
 
        kuap_unlock(regs->kuap, false);