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Date: Fri, 24 Jun 2022 14:57:48 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Will Deacon <will@...nel.org>
Cc: Linux ARM <linux-arm-kernel@...ts.infradead.org>,
linux-hardening@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Kees Cook <keescook@...omium.org>,
Catalin Marinas <catalin.marinas@....com>,
Mark Brown <broonie@...nel.org>,
Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v4 03/26] arm64: head: move assignment of idmap_t0sz to C code
On Fri, 24 Jun 2022 at 14:36, Will Deacon <will@...nel.org> wrote:
>
> On Mon, Jun 13, 2022 at 04:45:27PM +0200, Ard Biesheuvel wrote:
> > Setting idmap_t0sz involves fiddling with the caches if done with the
> > MMU off. Since we will be creating an initial ID map with the MMU and
> > caches off, and the permanent ID map with the MMU and caches on, let's
> > move this assignment of idmap_t0sz out of the startup code, and replace
> > it with a macro that simply issues the three instructions needed to
> > calculate the value wherever it is needed before the MMU is turned on.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > ---
> > arch/arm64/include/asm/assembler.h | 14 ++++++++++++++
> > arch/arm64/include/asm/mmu_context.h | 2 +-
> > arch/arm64/kernel/head.S | 13 +------------
> > arch/arm64/mm/mmu.c | 5 ++++-
> > arch/arm64/mm/proc.S | 2 +-
> > 5 files changed, 21 insertions(+), 15 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> > index 8c5a61aeaf8e..9468f45c07a6 100644
> > --- a/arch/arm64/include/asm/assembler.h
> > +++ b/arch/arm64/include/asm/assembler.h
> > @@ -359,6 +359,20 @@ alternative_cb_end
> > bfi \valreg, \t1sz, #TCR_T1SZ_OFFSET, #TCR_TxSZ_WIDTH
> > .endm
> >
> > +/*
> > + * idmap_get_t0sz - get the T0SZ value needed to cover the ID map
> > + *
> > + * Calculate the maximum allowed value for TCR_EL1.T0SZ so that the
> > + * entire ID map region can be mapped. As T0SZ == (64 - #bits used),
> > + * this number conveniently equals the number of leading zeroes in
> > + * the physical address of _end.
> > + */
> > + .macro idmap_get_t0sz, reg
> > + adrp \reg, _end
> > + orr \reg, \reg, #(1 << VA_BITS_MIN) - 1
> > + clz \reg, \reg
> > + .endm
> > +
> > /*
> > * tcr_compute_pa_size - set TCR.(I)PS to the highest supported
> > * ID_AA64MMFR0_EL1.PARange value
> > diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
> > index 6770667b34a3..6ac0086ebb1a 100644
> > --- a/arch/arm64/include/asm/mmu_context.h
> > +++ b/arch/arm64/include/asm/mmu_context.h
> > @@ -60,7 +60,7 @@ static inline void cpu_switch_mm(pgd_t *pgd, struct mm_struct *mm)
> > * TCR_T0SZ(VA_BITS), unless system RAM is positioned very high in
> > * physical memory, in which case it will be smaller.
> > */
> > -extern u64 idmap_t0sz;
> > +extern int idmap_t0sz;
> > extern u64 idmap_ptrs_per_pgd;
> >
> > /*
> > diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> > index dc07858eb673..7f361bc72d12 100644
> > --- a/arch/arm64/kernel/head.S
> > +++ b/arch/arm64/kernel/head.S
> > @@ -299,22 +299,11 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
> > * physical address space. So for the ID map, use an extended virtual
> > * range in that case, and configure an additional translation level
> > * if needed.
> > - *
> > - * Calculate the maximum allowed value for TCR_EL1.T0SZ so that the
> > - * entire ID map region can be mapped. As T0SZ == (64 - #bits used),
> > - * this number conveniently equals the number of leading zeroes in
> > - * the physical address of __idmap_text_end.
> > */
> > - adrp x5, __idmap_text_end
> > - clz x5, x5
> > + idmap_get_t0sz x5
> > cmp x5, TCR_T0SZ(VA_BITS_MIN) // default T0SZ small enough?
> > b.ge 1f // .. then skip VA range extension
> >
> > - adr_l x6, idmap_t0sz
> > - str x5, [x6]
> > - dmb sy
> > - dc ivac, x6 // Invalidate potentially stale cache line
> > -
> > #if (VA_BITS < 48)
> > #define EXTRA_SHIFT (PGDIR_SHIFT + PAGE_SHIFT - 3)
> > #define EXTRA_PTRS (1 << (PHYS_MASK_SHIFT - EXTRA_SHIFT))
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index 17b339c1a326..103bf4ae408d 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -43,7 +43,7 @@
> > #define NO_CONT_MAPPINGS BIT(1)
> > #define NO_EXEC_MAPPINGS BIT(2) /* assumes FEAT_HPDS is not used */
> >
> > -u64 idmap_t0sz = TCR_T0SZ(VA_BITS_MIN);
> > +int idmap_t0sz __ro_after_init;
> > u64 idmap_ptrs_per_pgd = PTRS_PER_PGD;
> >
> > #if VA_BITS > 48
> > @@ -785,6 +785,9 @@ void __init paging_init(void)
> > (u64)&vabits_actual + sizeof(vabits_actual));
> > #endif
> >
> > + idmap_t0sz = min(63UL - __fls(__pa_symbol(_end)),
> > + TCR_T0SZ(VA_BITS_MIN));
>
> nit: TCR_T0SZ shifts by TCR_T0SZ_OFFSET, so this is a bit confusing and
> works out because the register offset happens to be zero. Maybe it would
> be clearer to calculate the maximum of fls(__pa_symbol(_end)) and
> VA_BITS_MIN, then subtract that from 64?
>
I just noticed there are other inconsistencies with TCR_T0SZ(), e.g.,
in create_safe_exec_page(), which receives the 'shifted' value of
t0sz, but then shifts it again in cpu_install_ttbr0(). So this is
definitely
Let's just use the same expression as in the idmap_get_t0sz macro I am adding:
idmap_t0sz = 63UL - __fls(__pa_symbol(_end) | GENMASK(VA_BITS_MIN - 1, 0));
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