Re: [PATCH] target/i386: Fix physical address truncation when PAE is enabled

[Richard Henderson <richard.henderson@linaro.org>]

On 12/18/23 23:56, Michael Brown wrote:
> The address translation logic in get_physical_address() will currently
> truncate physical addresses to 32 bits unless long mode is enabled.
> This is incorrect when using physical address extensions (PAE) outside
> of long mode, with the result that a 32-bit operating system using PAE
> to access memory above 4G will experience undefined behaviour.
> 
> The truncation code was originally introduced in commit 33dfdb5 ("x86:
> only allow real mode to access 32bit without LMA"), where it applied
> only to translations performed while paging is disabled (and so cannot
> affect guests using PAE).
> 
> Commit 9828198 ("target/i386: Add MMU_PHYS_IDX and MMU_NESTED_IDX")
> rearranged the code such that the truncation also applied to the use
> of MMU_PHYS_IDX and MMU_NESTED_IDX.  Commit 4a1e9d4 ("target/i386: Use
> atomic operations for pte updates") brought this truncation into scope
> for page table entry accesses, and is the first commit for which a
> Windows 10 32-bit guest will reliably fail to boot if memory above 4G
> is present.
> 
> Fix by testing for PAE being enabled via the relevant bit in CR4,
> instead of testing for long mode being enabled.  PAE must be enabled
> as a prerequisite of long mode, and so this is a generalisation of the
> current test.
> 
> Remove the #ifdef TARGET_X86_64 check since PAE exists in both 32-bit
> and 64-bit processors, and both should exhibit the same truncation
> behaviour when PAE is disabled.
> 
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2040
> Signed-off-by: Michael Brown <mcb30@ipxe.org>
> ---
>   target/i386/tcg/sysemu/excp_helper.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/target/i386/tcg/sysemu/excp_helper.c b/target/i386/tcg/sysemu/excp_helper.c
> index 5b86f439ad..3d0d0d78d7 100644
> --- a/target/i386/tcg/sysemu/excp_helper.c
> +++ b/target/i386/tcg/sysemu/excp_helper.c
> @@ -582,12 +582,10 @@ static bool get_physical_address(CPUX86State *env, vaddr addr,
>   
>       /* Translation disabled. */
>       out->paddr = addr & x86_get_a20_mask(env);
> -#ifdef TARGET_X86_64
> -    if (!(env->hflags & HF_LMA_MASK)) {
> -        /* Without long mode we can only address 32bits in real mode */
> +    if (!(env->cr[4] & CR4_PAE_MASK)) {
> +        /* Without PAE we can address only 32 bits */
>           out->paddr = (uint32_t)out->paddr;
>       }
> -#endif

This is not the correct refactoring.

I agree that what we're currently doing is wrong, esp for MMU_PHYS_IDX, but for the 
default case, if CR0.PG == 0, then CR4.PAE is ignored (vol 3, section 4.1.1).

I suspect the correct fix is to have MMU_PHYS_IDX pass through the input address 
unchanged, and it is the responsibility of the higher level paging mmu_idx to truncate 
physical addresses per PG_MODE_* before recursing.


r~

r~


>       out->prot = PAGE_READ | PAGE_WRITE | PAGE_EXEC;
>       out->page_size = TARGET_PAGE_SIZE;
>       return true;