[xen staging] x86/EFI: Avoid mapping EFI system memory as shadow stacks

[patchbot@xxxxxxx]

commit 137d2d6310d31b4ace3df01832c37aa86b6173cc
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri May 29 21:49:13 2020 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri May 29 23:09:46 2020 +0100

    x86/EFI: Avoid mapping EFI system memory as shadow stacks
    
    Ensure the dirty bit is clear when creating read-only EFI mappings.
    
    Reported-by: Jan Beulich <jbeulich@xxxxxxxx>
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/common/efi/boot.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/common/efi/boot.c b/xen/common/efi/boot.c
index a6f84c945a..5a520bf21d 100644
--- a/xen/common/efi/boot.c
+++ b/xen/common/efi/boot.c
@@ -1554,7 +1554,7 @@ void __init efi_init_memory(void)
 
         if ( desc->Attribute & (efi_bs_revision < EFI_REVISION(2, 5)
                                 ? EFI_MEMORY_WP : EFI_MEMORY_RO) )
-            prot &= ~_PAGE_RW;
+            prot &= ~(_PAGE_DIRTY | _PAGE_RW);
         if ( desc->Attribute & EFI_MEMORY_XP )
             prot |= _PAGE_NX;
 
--
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