Patch "arm64: mm: limit linear region to 51 bits for KVM in nVHE mode" has been added to the 5.14-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    arm64: mm: limit linear region to 51 bits for KVM in nVHE mode

to the 5.14-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     arm64-mm-limit-linear-region-to-51-bits-for-kvm-in-n.patch
and it can be found in the queue-5.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit bb0dfd4b244b7529acec055e2e8d5020500a0406
Author: Ard Biesheuvel <ardb@xxxxxxxxxx>
Date:   Thu Aug 26 18:56:13 2021 +0200

    arm64: mm: limit linear region to 51 bits for KVM in nVHE mode
    
    [ Upstream commit 88053ec8cb1b91df566353cd3116470193797e00 ]
    
    KVM in nVHE mode divides up its VA space into two equal halves, and
    picks the half that does not conflict with the HYP ID map to map its
    linear region. This worked fine when the kernel's linear map itself was
    guaranteed to cover precisely as many bits of VA space, but this was
    changed by commit f4693c2716b35d08 ("arm64: mm: extend linear region for
    52-bit VA configurations").
    
    The result is that, depending on the placement of the ID map, kernel-VA
    to hyp-VA translations may produce addresses that either conflict with
    other HYP mappings (including the ID map itself) or generate addresses
    outside of the 52-bit addressable range, neither of which is likely to
    lead to anything useful.
    
    Given that 52-bit capable cores are guaranteed to implement VHE, this
    only affects configurations such as pKVM where we opt into non-VHE mode
    even if the hardware is VHE capable. So just for these configurations,
    let's limit the kernel linear map to 51 bits and work around the
    problem.
    
    Fixes: f4693c2716b3 ("arm64: mm: extend linear region for 52-bit VA configurations")
    Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
    Link: https://lore.kernel.org/r/20210826165613.60774-1-ardb@xxxxxxxxxx
    Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 1fdb7bb7c198..0ad4afc9359b 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -319,7 +319,21 @@ static void __init fdt_enforce_memory_region(void)
 
 void __init arm64_memblock_init(void)
 {
-	const s64 linear_region_size = PAGE_END - _PAGE_OFFSET(vabits_actual);
+	s64 linear_region_size = PAGE_END - _PAGE_OFFSET(vabits_actual);
+
+	/*
+	 * Corner case: 52-bit VA capable systems running KVM in nVHE mode may
+	 * be limited in their ability to support a linear map that exceeds 51
+	 * bits of VA space, depending on the placement of the ID map. Given
+	 * that the placement of the ID map may be randomized, let's simply
+	 * limit the kernel's linear map to 51 bits as well if we detect this
+	 * configuration.
+	 */
+	if (IS_ENABLED(CONFIG_KVM) && vabits_actual == 52 &&
+	    is_hyp_mode_available() && !is_kernel_in_hyp_mode()) {
+		pr_info("Capping linear region to 51 bits for KVM in nVHE mode on LVA capable hardware.\n");
+		linear_region_size = min_t(u64, linear_region_size, BIT(51));
+	}
 
 	/* Handle linux,usable-memory-range property */
 	fdt_enforce_memory_region();