Re: [Xen-devel] [PATCH] x86/msi: fix loop termination condition in pci_msi_conf_write_intercept()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/07/2019 10:47, Andrew Cooper wrote:
> On 02/07/2019 10:34, Paul Durrant wrote:
>> The for loop that deals with MSI masking is coded as follows:
>>
>> for ( pos = 0; pos < entry->msi.nvec; ++pos, ++entry )
>>
>> Thus the loop termination condition is dereferencing a struct pointer that
>> is being incremented by the loop. However, it is clear from following code
>> paths in msi_capability_init() that this is unsafe as for instance, in the
>> case of nvec == 1, entry will point at a single struct msi_desc allocation
>> and thus the loop will walk beyond the bounds of the allocation before
>> dereferencing the memory to determine whether the loop should terminate.
> More specifically, only entry[0].msi.nvec is correct.  All subsequent
> nvec fields are 0 in a block of entries.
>
>> Also, because the body of the loop writes via the entry pointer, this can
>> then lead to heap memory corruption, or indeed corruption of anything in
>> the direct map.
>>
>> This patch simply initializes a stack variable to the value of
>> entry->msi.nvec before starting the loop and then uses that in the
>> termination condition instead.

There is actually a second bug here which is being fixed.  How about
this for the commit message?

x86/msi: fix loop termination condition in
pci_msi_conf_write_intercept()                                                  
                                                                                
   

                                                                                
                                                                                
                                              

The for loop that deals with MSI masking is coded as
follows:                                                                        
                                                                         

                                                                                
                                                                                
                                              

for ( pos = 0; pos < entry->msi.nvec; ++pos, ++entry
)                                                                               
                                                                         

                                                                                
                                                                                
                                              

Thus the loop termination condition is dereferencing a struct pointer
that                                                                            
                                                        

is being incremented by the
loop.                                                                           
                                                                                
                  

                                                                                
                                                                                
                                              

A block of MSI entries stores the number of vectors in
entry[0].msi.nvec,                                                              
                                                                       

with all subsequent entries using a value of 0.  Therefore, for a block
of                                                                              
                                                      

two or more MSIs will terminate the loop early, as entry[1].msi.nvec is
0.                                                                              
                                                                  

                                                                                
                                                                                
                                              

However, for a single MSI, ++entry moves the pointer out of bounds, and
a                                                                               
                                                      

bogus read is used for the termination condition.  In the case that
the                                                                             
                                                          

loop body gets entered, there are subsequent OoB writes which
clobber                                                                         
                                                                

adjacent memory in the
heap.                                                                           
                                                                                
                       

                                                                                
                                                                                
                                              

This patch simply initializes a stack variable to the value
of                                                                              
                                                                  

entry->msi.nvec before starting the loop and then uses that in
the                                                                             
                                                               

termination condition instead.

~Andrew

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