[PATCH for 4.5] xen/arm: p2m: Fix crash when p2m_lookup is used with an invalid IPA

[PATCH for 4.5] xen/arm: p2m: Fix crash when p2m_lookup is used with an invalid IPA

[Julien Grall]

Since the commit 58f0fd8 "xen: arm: handle variable p2m levels in p2m_lookup",
Xen checks that the root_table offset is valid. If not, its unlock the p2m
spinlock before returning an error. But, at this time, the lock has not been
taken.

On Xen built with debug=y, we can get the following stack trace if the guest
use an invalid IPA in hypercall or mess-up the grant-table:

(XEN) Assertion '_raw_spin_is_locked(lock)' failed at xen/include/asm/arm32/spinlock.h:22
...
(XEN)    [<0022d1bc>] _spin_unlock+0x2c/0x50 (PC)
(XEN)    [<00253264>] p2m_lookup+0x20c/0x230 (LR)
(XEN)    [<7ffdfd54>] 7ffdfd54
(XEN)    [<002539f4>] gmfn_to_mfn+0x24/0x3c
(XEN)    [<0020e4d4>] __get_paged_frame+0x30/0x12c
(XEN)    [<00210680>] __acquire_grant_for_copy+0x4e0/0x768
(XEN)    [<00212030>] do_grant_table_op+0x13a0/0x2534
(XEN)    [<00257b10>] do_trap_hypervisor+0xe10/0x1148
(XEN)    [<0025b330>] return_from_trap+0/0x4

Signed-off-by: Julien Grall <julien.grall@linaro.org>
---
    This is a bug fix for Xen 4.5. Any buggy guest could make Xen crash in
    debug build. I haven't really though about what could happen in
    non-debug build.
---
 xen/arch/arm/p2m.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
index 1585d35..69191b9 100644
--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -207,9 +207,8 @@ paddr_t p2m_lookup(struct domain *d, paddr_t paddr, p2m_type_t *t)
         *t = pte.p2m.type;
     }
 
-err:
     spin_unlock(&p2m->lock);
-
+err:
     return maddr;
 }
 
-- 
1.7.10.4

Re: [PATCH for 4.5] xen/arm: p2m: Fix crash when p2m_lookup is used with an invalid IPA

[Ian Campbell]

On Sat, 2014-10-18 at 20:25 +0100, Julien Grall wrote:
> Since the commit 58f0fd8 "xen: arm: handle variable p2m levels in p2m_lookup",
> Xen checks that the root_table offset is valid. If not, its unlock the p2m
> spinlock before returning an error. But, at this time, the lock has not been
> taken.
> 
> On Xen built with debug=y, we can get the following stack trace if the guest
> use an invalid IPA in hypercall or mess-up the grant-table:
> 
> (XEN) Assertion '_raw_spin_is_locked(lock)' failed at xen/include/asm/arm32/spinlock.h:22
> ...
> (XEN)    [<0022d1bc>] _spin_unlock+0x2c/0x50 (PC)
> (XEN)    [<00253264>] p2m_lookup+0x20c/0x230 (LR)
> (XEN)    [<7ffdfd54>] 7ffdfd54
> (XEN)    [<002539f4>] gmfn_to_mfn+0x24/0x3c
> (XEN)    [<0020e4d4>] __get_paged_frame+0x30/0x12c
> (XEN)    [<00210680>] __acquire_grant_for_copy+0x4e0/0x768
> (XEN)    [<00212030>] do_grant_table_op+0x13a0/0x2534
> (XEN)    [<00257b10>] do_trap_hypervisor+0xe10/0x1148
> (XEN)    [<0025b330>] return_from_trap+0/0x4
> 
> Signed-off-by: Julien Grall <julien.grall@linaro.org>

acked + applied, thanks.