Re: [PATCH] irqchip/gic-v3-its: zero ITS tables in the shared view after decryption

[Marc Zyngier]

On Mon, 24 Aug 2026 16:59:27 +0100,
Shanker Donthineni <sdonthineni@xxxxxxxxxx> wrote:
> 
> its_alloc_pages_node() allocates pages, some with __GFP_ZERO, before
> calling set_memory_decrypted() to share them with the untrusted
> hypervisor. In an Arm CCA Realm, allocator clearing occurs through the
> Protected IPA.
> 
> set_memory_decrypted() changes the RIPAS of the Protected IPA from
> RIPAS_RAM to RIPAS_EMPTY. The RMM architecture requires the backing
> DATA Granule to be wiped before it becomes GRAN_UNDELEGATED. Wiping is
> a confidentiality operation, not a zero-initialization guarantee: it
> may be implemented by changing the MEC or writing random data.
> Consequently, zeroes written through the Protected IPA are not
> guaranteed to be observed through the corresponding Unprotected IPA
> alias.
> 
> This is harmless for tables that the guest fully populates before use,
> but the GICv3 ITS indirect device table is sparse. The guest writes only
> the L1 entries for devices it maps, while both the guest and the
> hypervisor's ITS emulation rely on unused entries being zero.
> Unspecified nonzero values can make an unpopulated entry appear valid,
> causing the guest to skip the required L2 allocation and the hypervisor
> to consume a bogus pointer. Interrupt translation then fails,
> potentially hanging the guest when it first uses the device. This is
> exposed when guest_memfd reuses the same backing Granule for the
> Unprotected IPA mapping rather than supplying a fresh Host page.
> 
> Strip __GFP_ZERO before allocation and clear the page through the
> Unprotected IPA alias after set_memory_decrypted(), so the requested
> zeroes are written in Non-secure PAS and are visible to both the guest
> and the Host. This also matches dma_direct_alloc(), which clears memory
> after dma_set_decrypted().
> 
> Fixes: b08e2f42e86b ("irqchip/gic-v3-its: Share ITS tables with a non-trusted hypervisor")
> Cc: <stable@xxxxxxxxxxxxxxx> # 6.18+
> Signed-off-by: Shanker Donthineni <sdonthineni@xxxxxxxxxx>

https://lore.kernel.org/r/20260820105026.53208-1-steven.price@xxxxxxx

	M.

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