Re: [Xen-devel] [PATCH] arm: p2m.c bug-fix: hypervisor hang on __p2m_get_mem_access

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Tue, 2016-01-26 at 13:46 +0200, Corneliu ZUZU wrote:
> When __p2m_get_mem_access gets called, the p2m lock is already taken
> by either get_page_from_gva or p2m_get_mem_access.
> 
> Possible code paths:
> 1)    -> get_page_from_gva
>               -> p2m_mem_access_check_and_get_page
>                       -> __p2m_get_mem_access
> 2)    -> p2m_get_mem_access
>               -> __p2m_get_mem_access

What about:
        -> p2m_mem_access_check
                ->Âp2m_get_mem_access
I can't see the lock being taken in that paths.

As well as fixing that I think it would be wise to add an assert that the
lock is held before the call to p2m_lookup which you are changing into the
unlocked variant.

> 
> In both cases if __p2m_get_mem_access subsequently gets to
> call p2m_lookup (happens if !radix_tree_lookup(...)), a hypervisor
> hang will occur, since p2m_lookup also spin-locks on the p2m lock.
> 
> This bug-fix simply replaces the p2m_lookup call from
> __p2m_get_mem_access
> with a call to __p2m_lookup.
> 
> Signed-off-by: Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>
> ---
> Âxen/arch/arm/p2m.c | 2 +-
> Â1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 2190908..a9157e5 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -490,7 +490,7 @@ static int __p2m_get_mem_access(struct domain *d,
> gfn_t gfn,
> ÂÂÂÂÂÂÂÂÂÂ* No setting was found in the Radix tree. Check if the
> ÂÂÂÂÂÂÂÂÂÂ* entry exists in the page-tables.
> ÂÂÂÂÂÂÂÂÂÂ*/
> -ÂÂÂÂÂÂÂÂpaddr_t maddr = p2m_lookup(d, gfn_x(gfn) << PAGE_SHIFT, NULL);
> +ÂÂÂÂÂÂÂÂpaddr_t maddr = __p2m_lookup(d, gfn_x(gfn) << PAGE_SHIFT, NULL);
> ÂÂÂÂÂÂÂÂÂif ( INVALID_PADDR == maddr )
> ÂÂÂÂÂÂÂÂÂÂÂÂÂreturn -ESRCH;
> Â

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