netdev - Re: [PATCH 3/3] arch/arm64/kernel/traps: Use find_vma_intersection() in traps for setting si

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Message-ID: <20210422125334.GB1521@willie-the-truck>
Date:   Thu, 22 Apr 2021 13:53:34 +0100
From:   Will Deacon <will@...nel.org>
To:     Liam Howlett <liam.howlett@...cle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Julien Grall <julien.grall@....com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCH 3/3] arch/arm64/kernel/traps: Use find_vma_intersection()
 in traps for setting si_code

On Tue, Apr 20, 2021 at 04:50:13PM +0000, Liam Howlett wrote:
> find_vma() will continue to search upwards until the end of the virtual
> memory space.  This means the si_code would almost never be set to
> SEGV_MAPERR even when the address falls outside of any VMA.  The result
> is that the si_code is not reliable as it may or may not be set to the
> correct result, depending on where the address falls in the address
> space.
> 
> Using find_vma_intersection() allows for what is intended by only
> returning a VMA if it falls within the range provided, in this case a
> window of 1.
> 
> Fixes: bd35a4adc413 (arm64: Port SWP/SWPB emulation support from arm)
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
>  arch/arm64/kernel/traps.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index a05d34f0e82a..a44007904a64 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -383,9 +383,10 @@ void force_signal_inject(int signal, int code, unsigned long address, unsigned i
>  void arm64_notify_segfault(unsigned long addr)
>  {
>  	int code;
> +	unsigned long ut_addr = untagged_addr(addr);
>  
>  	mmap_read_lock(current->mm);
> -	if (find_vma(current->mm, untagged_addr(addr)) == NULL)
> +	if (find_vma_intersection(current->mm, ut_addr, ut_addr + 1) == NULL)
>  		code = SEGV_MAPERR;
>  	else
>  		code = SEGV_ACCERR;

I'm not seeing how this addresses VM_GROWSDOWN as Catalin mentioned before.

Will