linux-kernel - Re: [PATCH v2] arm64: kaslr: Fix up the kernel image alignment

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Date:   Wed, 22 Mar 2017 12:40:08 +0000
From:   Will Deacon <will.deacon@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Srinivas Ramana <sramana@...eaurora.org>,
        Catalin Marinas <catalin.marinas@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-arm-msm@...r.kernel.org,
        Neeraj Upadhyay <neeraju@...eaurora.org>
Subject: Re: [PATCH v2] arm64: kaslr: Fix up the kernel image alignment

On Wed, Mar 22, 2017 at 12:16:24PM +0000, Ard Biesheuvel wrote:
> On 22 March 2017 at 11:38, Srinivas Ramana <sramana@...eaurora.org> wrote:
> > From: Neeraj Upadhyay <neeraju@...eaurora.org>
> >
> > If kernel image extends across alignment boundary, existing
> > code increases the KASLR offset by size of kernel image. The
> > offset is masked after resizing. There are cases, where after
> > masking, we may still have kernel image extending across
> > boundary. This eventually results in only 2MB block getting
> > mapped while creating the page tables. This results in data aborts
> > while accessing unmapped regions during second relocation (with
> > kaslr offset) in __primary_switch. To fix this problem, round up the
> > kernel image size, by swapper block size, before adding it for
> > correction.
> >
> > For example consider below case, where kernel image still crosses
> > 1GB alignment boundary, after masking the offset, which is fixed
> > by rounding up kernel image size.
> >
> > SWAPPER_TABLE_SHIFT = 30
> > Swapper using section maps with section size 2MB.
> > CONFIG_PGTABLE_LEVELS = 3
> > VA_BITS = 39
> >
> > _text  : 0xffffff8008080000
> > _end   : 0xffffff800aa1b000
> > offset : 0x1f35600000
> > mask = ((1UL << (VA_BITS - 2)) - 1) & ~(SZ_2M - 1)
> >
> > (_text + offset) >> SWAPPER_TABLE_SHIFT = 0x3fffffe7c
> > (_end + offset) >> SWAPPER_TABLE_SHIFT  = 0x3fffffe7d
> >
> > offset after existing correction (before mask) = 0x1f37f9b000
> > (_text + offset) >> SWAPPER_TABLE_SHIFT = 0x3fffffe7d
> > (_end + offset) >> SWAPPER_TABLE_SHIFT  = 0x3fffffe7d
> >
> > offset (after mask) = 0x1f37e00000
> > (_text + offset) >> SWAPPER_TABLE_SHIFT = 0x3fffffe7c
> > (_end + offset) >> SWAPPER_TABLE_SHIFT  = 0x3fffffe7d
> >
> > new offset w/ rounding up = 0x1f38000000
> > (_text + offset) >> SWAPPER_TABLE_SHIFT = 0x3fffffe7d
> > (_end + offset) >> SWAPPER_TABLE_SHIFT  = 0x3fffffe7d
> >
> > Fixes: f80fb3a3d508 ("arm64: add support for kernel ASLR")
> > Signed-off-by: Neeraj Upadhyay <neeraju@...eaurora.org>
> > Signed-off-by: Srinivas Ramana <sramana@...eaurora.org>
> 
> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> 
> ... and thanks for the excellent commit log message!

Thanks both. I've picked this up as a fix.

Will