linux-kernel - [PATCH] KVM: X86: prevent integer overflows in KVM_MEMORY_ENCRYPT_REG

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Message-ID: <20180519060135.GA4991@mwanda>
Date:   Sat, 19 May 2018 09:01:36 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: [PATCH] KVM: X86: prevent integer overflows in
 KVM_MEMORY_ENCRYPT_REG_REGION

This is a fix from reviewing the code, but it looks like it might be
able to lead to an Oops.  It affects 32bit systems.

The KVM_MEMORY_ENCRYPT_REG_REGION ioctl uses a u64 for range->addr and
range->size but the high 32 bits would be truncated away on a 32 bit
system.  This is harmless but it's also harmless to prevent it.

Then in sev_pin_memory() the "uaddr + ulen" calculation can wrap around.
The wrap around can happen on 32 bit or 64 bit systems, but I was only
able to figure out a problem for 32 bit systems.  We would pick a number
which results in "npages" being zero.  The sev_pin_memory() would then
return ZERO_SIZE_PTR without allocating anything.

I made it illegal to call sev_pin_memory() with "ulen" set to zero.
Hopefully, that doesn't cause any problems.  I also changed the type of
"first" and "last" to long, just for cosmetic reasons.  Otherwise on a
64 bit system you're saving "uaddr >> 12" in an int and it truncates the
high 20 bits away.  The math works in the current code so far as I can
see but it's just weird.

Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
---
Again, this is a static checker fix.  The most risky parts of this
patch are blocking "ulen == 0" and changing the types of "first" and
"last".  I felt like those changes made the math easier to understand

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 220e5a89465a..de21d5c5168b 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1762,7 +1762,10 @@ static struct page **sev_pin_memory(struct kvm *kvm, unsigned long uaddr,
 	unsigned long npages, npinned, size;
 	unsigned long locked, lock_limit;
 	struct page **pages;
-	int first, last;
+	unsigned long first, last;
+
+	if (ulen == 0 || uaddr + ulen < uaddr)
+		return NULL;

 	/* Calculate number of pages. */
 	first = (uaddr & PAGE_MASK) >> PAGE_SHIFT;
@@ -6925,6 +6928,9 @@ static int svm_register_enc_region(struct kvm *kvm,
 	if (!sev_guest(kvm))
 		return -ENOTTY;

+	if (range->addr > ULONG_MAX || range->size > ULONG_MAX)
+		return -EINVAL;
+
 	region = kzalloc(sizeof(*region), GFP_KERNEL);
 	if (!region)
 		return -ENOMEM;