linux-kernel - Re: [PATCH V2 1/4] arm64/mm: Drop mmap_sem before calling __do_kernel

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Date:   Tue, 4 Jun 2019 15:44:51 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH V2 1/4] arm64/mm: Drop mmap_sem before calling
 __do_kernel_fault()

On Mon, Jun 03, 2019 at 12:11:22PM +0530, Anshuman Khandual wrote:
> There is an inconsistency between down_read_trylock() success and failure
> paths while dealing with kernel access for non exception table areas where
> it calls __do_kernel_fault(). In case of failure it just bails out without
> holding mmap_sem but when it succeeds it does so while holding mmap_sem.
> Fix this inconsistency by just dropping mmap_sem in success path as well.
> 
> __do_kernel_fault() calls die_kernel_fault() which then calls show_pte().
> show_pte() in this path might become bit more unreliable without holding
> mmap_sem. But there are already instances [1] in do_page_fault() where
> die_kernel_fault() gets called without holding mmap_sem. show_pte() can
> be made more robust independently but in a later patch.
> 
> [1] Conditional block for (is_ttbr0_addr && is_el1_permission_fault)
> 
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: James Morse <james.morse@....com>
> Cc: Andrey Konovalov <andreyknvl@...gle.com>

Queued for 5.3. Thanks.

-- 
Catalin