[xen stable-4.17] x86/shadow: restore dropped check in sh_unshadow_for_p2m_change()

[patchbot@xxxxxxx]

commit 8b5be1fe938f52b5d3682dee7702fd51c8cfb61b
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Tue Apr 25 15:47:44 2023 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Apr 25 15:47:44 2023 +0200

    x86/shadow: restore dropped check in sh_unshadow_for_p2m_change()
    
    As a result of 241702e064604dbb3e0d9b731aa8f45be448243b the
    mfn_valid() check in sh_unshadow_for_p2m_change() was lost.  That
    allows sh_remove_shadows() to be called with gfns that have no backing
    page, causing an ASSERT to trigger in debug builds or dereferencing an
    arbitrary pointer partially under guest control in non-debug builds:
    
    RIP:    e008:[<ffff82d0402dcf2c>] sh_remove_shadows+0x19f/0x722
    RFLAGS: 0000000000010246   CONTEXT: hypervisor (d0v2)
    [...]
    Xen call trace:
       [<ffff82d0402dcf2c>] R sh_remove_shadows+0x19f/0x722
       [<ffff82d0402e28f4>] F 
arch/x86/mm/shadow/hvm.c#sh_unshadow_for_p2m_change+0xab/0x2b7
       [<ffff82d040311931>] F arch/x86/mm/p2m-pt.c#write_p2m_entry+0x19b/0x4d3
       [<ffff82d0403131b2>] F arch/x86/mm/p2m-pt.c#p2m_pt_set_entry+0x67b/0xa8e
       [<ffff82d040302c92>] F p2m_set_entry+0xcc/0x149
       [<ffff82d040305a50>] F unmap_mmio_regions+0x17b/0x2c9
       [<ffff82d040241e5e>] F do_domctl+0x11f3/0x195e
       [<ffff82d0402c7e10>] F hvm_hypercall+0x5b1/0xa2d
       [<ffff82d0402adc72>] F vmx_vmexit_handler+0x130f/0x1cd5
       [<ffff82d040203602>] F vmx_asm_vmexit_handler+0xf2/0x210
    
    ****************************************
    Panic on CPU 1:
    Assertion 'mfn_valid(gmfn)' failed at arch/x86/mm/shadow/common.c:2203
    ****************************************
    
    Fix this by restoring the mfn_valid() check in
    sh_unshadow_for_p2m_change(), unifying it with the rest of the checks
    that are done at the start of the function.
    
    This is XSA-430 / CVE-2022-42335
    
    Fixes: 241702e064 ('x86/shadow: slightly consolidate 
sh_unshadow_for_p2m_change() (part II)')
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: f6c3cb21628f7bed73cb992da400f6b36630f290
    master date: 2023-04-25 15:44:54 +0200
---
 xen/arch/x86/mm/shadow/hvm.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/mm/shadow/hvm.c b/xen/arch/x86/mm/shadow/hvm.c
index 29a58d9131..39abf4732d 100644
--- a/xen/arch/x86/mm/shadow/hvm.c
+++ b/xen/arch/x86/mm/shadow/hvm.c
@@ -814,7 +814,8 @@ static void cf_check sh_unshadow_for_p2m_change(
 
     /* Only previously present / valid entries need processing. */
     if ( !(oflags & _PAGE_PRESENT) ||
-         (!p2m_is_valid(p2mt) && !p2m_is_grant(p2mt)) )
+         (!p2m_is_valid(p2mt) && !p2m_is_grant(p2mt)) ||
+         !mfn_valid(omfn) )
         return;
 
     switch ( level )
--
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