Re: [Xen-devel] xen/arm: potential bug in advance_pc

[Julien Grall <julien.grall@xxxxxxx>]

On 14/05/2019 17:03, Lukas Jünger wrote:
> Hello all,
Hello,

Thank you for the report.

Please try to CC the maintainers of the code (in this case Stefano and I) when 
asking a question. So your e-mail don't get lost in the noise of xen-devel.
> in the function advance_pc in xen/arch/arm/traps.c in line 1655,1656 you can 
> find the following code:
> 1655     BUG_ON( (!psr_mode_is_32bit(cpsr)||!(cpsr&PSR_THUMB))
> 1656             && (cpsr&PSR_IT_MASK) );
>
> This code seems to check that we are not running in thumb mode and that the 
> PSR_IT_MASK is not set.
> On ARMv8.5-BTI systems bits [11:10] of spsr_el2 indicate the BTYPE (see 
> https://developer.arm.com/docs/ddi0595/b/aarch64-system-registers/spsr_el2).
> If an exception is taken in the guest (e.g. write to system register) from 
> AArch64 state these bits might be set.
> The PSR_IT_MASK for thumb mode overlaps with these bits and BUG_ON is executed.
> This seems to be a bug.
Xen code blindly assumed that RES0 means the bits cannot be used for other 
purpose in later revision of the architecture. So this is clearly a bug.
> Is it really necessary to check the PSR_IT_MASK for BUG_ON here?
No, and it is a pretty bad idea to check guest architectural behavior with 
BUG_ON(). The BUG_ON() needs to be replaced with proper check on the CPSR (see 
below).
> Why is the execution mode checked twice with psr_mode_is_32bit and 
> cpsr&PSR_THUMB, as they seem to do the same thing?
32-bit mode support two instruction set: ARM and Thumb. From my understanding 
the value of IT is unknown when using the ARM instruction set. Furthermore, the 
bit 5 only means Thumb on 32-bit mode. For 64-bit mode, the bit is RES0.
So we would need to check the 2 bits.

> If PSR_IT_MASK does not need to be checked for BUG_ON, the if statement in the 
> following line should check for thumb mode again, right?
The check should be something like:

if ( psr_mode_is_32bit(cpsr) && (cpsr & PSR_THUMB) && (cpsr & PSR_IT_MASK) )
{
}

I have noticed few more issues in the traps.c code regarding RES0 . I will send 
a series with that fixed later today.
Cheers,

--
Julien Grall

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