6.8-stable review patch. If anyone has any objections, please let me know.
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From: Ard Biesheuvel ardb@kernel.org
[ Upstream commit a0025f587c685e5ff842fb0194036f2ca0b6eaf4 ]
The early 64-bit boot code must be entered with a 1:1 mapping of the bootable image, but it cannot operate without a 1:1 mapping of all the assets in memory that it accesses, and therefore, it creates such mappings for all known assets upfront, and additional ones on demand when a page fault happens on a memory address.
These mappings are created with the global bit G set, as the flags used to create page table descriptors are based on __PAGE_KERNEL_LARGE_EXEC defined by the core kernel, even though the context where these mappings are used is very different.
This means that the TLB maintenance carried out by the decompressor is not sufficient if it is entered with CR4.PGE enabled, which has been observed to happen with the stage0 bootloader of project Oak. While this is a dubious practice if no global mappings are being used to begin with, the decompressor is clearly at fault here for creating global mappings and not performing the appropriate TLB maintenance.
Since commit:
f97b67a773cd84b ("x86/decompressor: Only call the trampoline when changing paging levels")
CR4 is no longer modified by the decompressor if no change in the number of paging levels is needed. Before that, CR4 would always be set to a consistent value with PGE cleared.
So let's reinstate a simplified version of the original logic to put CR4 into a known state, and preserve the PAE, MCE and LA57 bits, none of which can be modified freely at this point (PAE and LA57 cannot be changed while running in long mode, and MCE cannot be cleared when running under some hypervisors).
This effectively clears PGE and works around the project Oak bug.
Fixes: f97b67a773cd84b ("x86/decompressor: Only call the trampoline when ...") Signed-off-by: Ard Biesheuvel ardb@kernel.org Signed-off-by: Ingo Molnar mingo@kernel.org Cc: Linus Torvalds torvalds@linux-foundation.org Cc: "H. Peter Anvin" hpa@zytor.com Link: https://lore.kernel.org/r/20240410151354.506098-2-ardb+git@google.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/boot/compressed/head_64.S | 5 +++++ 1 file changed, 5 insertions(+)
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S index bf4a10a5794f1..1dcb794c5479e 100644 --- a/arch/x86/boot/compressed/head_64.S +++ b/arch/x86/boot/compressed/head_64.S @@ -398,6 +398,11 @@ SYM_CODE_START(startup_64) call sev_enable #endif
+ /* Preserve only the CR4 bits that must be preserved, and clear the rest */ + movq %cr4, %rax + andl $(X86_CR4_PAE | X86_CR4_MCE | X86_CR4_LA57), %eax + movq %rax, %cr4 + /* * configure_5level_paging() updates the number of paging levels using * a trampoline in 32-bit addressable memory if the current number does