x86/sev-es: Allow copy_from_kernel_nofault() in earlier boot

[ Upstream commit f79936545fb122856bd78b189d3c7ee59928c751 ]

Previously, if copy_from_kernel_nofault() was called before
boot_cpu_data.x86_virt_bits was set up, then it would trigger undefined
behavior due to a shift by 64.

This ended up causing boot failures in the latest version of ubuntu2204
in the gcp project when using SEV-SNP.

Specifically, this function is called during an early #VC handler which
is triggered by a CPUID to check if NX is implemented.

Fixes: 1aa9aa8ee517 ("x86/sev-es: Setup GHCB-based boot #VC handler")
Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Adam Dunlap <acdunlap@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Jacob Xu <jacobhxu@google.com>
Link: https://lore.kernel.org/r/20230912002703.3924521-2-acdunlap@google.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/arch/x86/mm/maccess.c b/arch/x86/mm/maccess.c
index 5a53c2c..6993f02 100644 (file)
--- a/arch/x86/mm/maccess.c
+++ b/arch/x86/mm/maccess.c
@@ -9,12 +9,21 @@ bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
        unsigned long vaddr = (unsigned long)unsafe_src;
 
        /*
-        * Range covering the highest possible canonical userspace address
-        * as well as non-canonical address range. For the canonical range
-        * we also need to include the userspace guard page.
+        * Do not allow userspace addresses.  This disallows
+        * normal userspace and the userspace guard page:
         */
-       return vaddr >= TASK_SIZE_MAX + PAGE_SIZE &&
-              __is_canonical_address(vaddr, boot_cpu_data.x86_virt_bits);
+       if (vaddr < TASK_SIZE_MAX + PAGE_SIZE)
+               return false;
+
+       /*
+        * Allow everything during early boot before 'x86_virt_bits'
+        * is initialized.  Needed for instruction decoding in early
+        * exception handlers.
+        */
+       if (!boot_cpu_data.x86_virt_bits)
+               return true;
+
+       return __is_canonical_address(vaddr, boot_cpu_data.x86_virt_bits);
 }
 #else
 bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)