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Date: Mon, 13 May 2024 11:41:34 -0700
From: Deepak Gupta <debug@rivosinc.com>
To: Charlie Jenkins <charlie@rivosinc.com>
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Subject: Re: [PATCH v3 10/29] riscv/mm : ensure PROT_WRITE leads to VM_READ |
 VM_WRITE
Message-ID: <ZkJe3ivq7m4NptHd@debug.ba.rivosinc.com>
References: <20240403234054.2020347-1-debug@rivosinc.com>
 <20240403234054.2020347-11-debug@rivosinc.com>
 <Zj6LfpQhOjTLEx2O@ghost>
 <ZkJSLTk1iWFGJZCQ@debug.ba.rivosinc.com>
 <ZkJdYvkUqHkX7yPf@ghost>
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In-Reply-To: <ZkJdYvkUqHkX7yPf@ghost>

On Mon, May 13, 2024 at 11:36:49AM -0700, Charlie Jenkins wrote:
>On Mon, May 13, 2024 at 10:47:25AM -0700, Deepak Gupta wrote:
>> On Fri, May 10, 2024 at 02:02:54PM -0700, Charlie Jenkins wrote:
>> > On Wed, Apr 03, 2024 at 04:34:58PM -0700, Deepak Gupta wrote:
>> > > `arch_calc_vm_prot_bits` is implemented on risc-v to return VM_READ |
>> > > VM_WRITE if PROT_WRITE is specified. Similarly `riscv_sys_mmap` is
>> > > updated to convert all incoming PROT_WRITE to (PROT_WRITE | PROT_READ).
>> > > This is to make sure that any existing apps using PROT_WRITE still work.
>> > >
>> > > Earlier `protection_map[VM_WRITE]` used to pick read-write PTE encodings.
>> > > Now `protection_map[VM_WRITE]` will always pick PAGE_SHADOWSTACK PTE
>> > > encodings for shadow stack. Above changes ensure that existing apps
>> > > continue to work because underneath kernel will be picking
>> > > `protection_map[VM_WRITE|VM_READ]` PTE encodings.
>> > >
>> > > Signed-off-by: Deepak Gupta <debug@rivosinc.com>
>> > > ---
>> > >  arch/riscv/include/asm/mman.h    | 24 ++++++++++++++++++++++++
>> > >  arch/riscv/include/asm/pgtable.h |  1 +
>> > >  arch/riscv/kernel/sys_riscv.c    | 11 +++++++++++
>> > >  arch/riscv/mm/init.c             |  2 +-
>> > >  mm/mmap.c                        |  1 +
>> > >  5 files changed, 38 insertions(+), 1 deletion(-)
>> > >  create mode 100644 arch/riscv/include/asm/mman.h
>> > >
>> > > diff --git a/arch/riscv/include/asm/mman.h b/arch/riscv/include/asm/mman.h
>> > > new file mode 100644
>> > > index 000000000000..ef9fedf32546
>> > > --- /dev/null
>> > > +++ b/arch/riscv/include/asm/mman.h
>> > > @@ -0,0 +1,24 @@
>> > > +/* SPDX-License-Identifier: GPL-2.0 */
>> > > +#ifndef __ASM_MMAN_H__
>> > > +#define __ASM_MMAN_H__
>> > > +
>> > > +#include <linux/compiler.h>
>> > > +#include <linux/types.h>
>> > > +#include <uapi/asm/mman.h>
>> > > +
>> > > +static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot,
>> > > +	unsigned long pkey __always_unused)
>> > > +{
>> > > +	unsigned long ret = 0;
>> > > +
>> > > +	/*
>> > > +	 * If PROT_WRITE was specified, force it to VM_READ | VM_WRITE.
>> > > +	 * Only VM_WRITE means shadow stack.
>> > > +	 */
>> > > +	if (prot & PROT_WRITE)
>> > > +		ret = (VM_READ | VM_WRITE);
>> > > +	return ret;
>> > > +}
>> > > +#define arch_calc_vm_prot_bits(prot, pkey) arch_calc_vm_prot_bits(prot, pkey)
>> > > +
>> > > +#endif /* ! __ASM_MMAN_H__ */
>> > > diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
>> > > index 6066822e7396..4d5983bc6766 100644
>> > > --- a/arch/riscv/include/asm/pgtable.h
>> > > +++ b/arch/riscv/include/asm/pgtable.h
>> > > @@ -184,6 +184,7 @@ extern struct pt_alloc_ops pt_ops __initdata;
>> > >  #define PAGE_READ_EXEC		__pgprot(_PAGE_BASE | _PAGE_READ | _PAGE_EXEC)
>> > >  #define PAGE_WRITE_EXEC		__pgprot(_PAGE_BASE | _PAGE_READ |	\
>> > >  					 _PAGE_EXEC | _PAGE_WRITE)
>> > > +#define PAGE_SHADOWSTACK       __pgprot(_PAGE_BASE | _PAGE_WRITE)
>> > >
>> > >  #define PAGE_COPY		PAGE_READ
>> > >  #define PAGE_COPY_EXEC		PAGE_READ_EXEC
>> > > diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
>> > > index f1c1416a9f1e..846c36b1b3d5 100644
>> > > --- a/arch/riscv/kernel/sys_riscv.c
>> > > +++ b/arch/riscv/kernel/sys_riscv.c
>> > > @@ -8,6 +8,8 @@
>> > >  #include <linux/syscalls.h>
>> > >  #include <asm/cacheflush.h>
>> > >  #include <asm-generic/mman-common.h>
>> > > +#include <vdso/vsyscall.h>
>> > > +#include <asm/mman.h>
>> > >
>> > >  static long riscv_sys_mmap(unsigned long addr, unsigned long len,
>> > >  			   unsigned long prot, unsigned long flags,
>> > > @@ -17,6 +19,15 @@ static long riscv_sys_mmap(unsigned long addr, unsigned long len,
>> > >  	if (unlikely(offset & (~PAGE_MASK >> page_shift_offset)))
>> > >  		return -EINVAL;
>> > >
>> > > +	/*
>> > > +	 * If only PROT_WRITE is specified then extend that to PROT_READ
>> > > +	 * protection_map[VM_WRITE] is now going to select shadow stack encodings.
>> > > +	 * So specifying PROT_WRITE actually should select protection_map [VM_WRITE | VM_READ]
>> > > +	 * If user wants to create shadow stack then they should use `map_shadow_stack` syscall.
>> > > +	 */
>> > > +	if (unlikely((prot & PROT_WRITE) && !(prot & PROT_READ)))
>> >
>> > The comments says that this should extend to PROT_READ if only
>> > PROT_WRITE is specified. This condition instead is checking if
>> > PROT_WRITE is selected but PROT_READ is not. If prot is (VM_EXEC |
>> > VM_WRITE) then it would be extended to (VM_EXEC | VM_WRITE | VM_READ).
>> > This will not currently cause any issues because these both map to the
>> > same value in the protection_map PAGE_COPY_EXEC, however this seems to
>> > be not the intention of this change.
>> >
>> > prot == PROT_WRITE better suits the condition explained in the comment.
>>
>> If someone specifies this (PROT_EXEC | PROT_WRITE) today, it works because
>> of the way permissions are setup in `protection_map`. On risc-v there is no
>> way to have a page which is execute and write only. So expectation is that
>> if some apps were using `PROT_EXEC | PROT_WRITE` today, they were working
>> because internally it was translating to read, write and execute on page
>> permissions level. This patch make sure that, it stays same from page
>> permissions perspective.
>>
>> If someone was using PROT_EXEC, it may translate to execute only and this change
>> doesn't impact that.
>>
>> Patch simply looks for presence of `PROT_WRITE` and absence of `PROT_READ` in
>> protection flags and if that condition is satisfied, it assumes that caller assumed
>> page is going to be read allowed as well.
>
>The purpose of this change is for compatibility with shadow stack pages
>but this affects flags for pages that are not shadow stack pages.
>Adding PROT_READ to the other cases is redundant as protection_map
>already handles that mapping. Permissions being strictly PROT_WRITE is
>the only case that needs to be handled, and is the only case that is
>called out in the commit message and in the comment.

Yeah that's fine.
I can change the commit message or just strictly check for PROT_WRITE.
It doesn't change bottomline, I am fine with either option.

Let me know your preference.

>
>- Charlie
>
>>
>>
>> >
>> > > +		prot |= PROT_READ;
>> > > +
>> > >  	return ksys_mmap_pgoff(addr, len, prot, flags, fd,
>> > >  			       offset >> (PAGE_SHIFT - page_shift_offset));
>> > >  }
>> > > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>> > > index fa34cf55037b..98e5ece4052a 100644
>> > > --- a/arch/riscv/mm/init.c
>> > > +++ b/arch/riscv/mm/init.c
>> > > @@ -299,7 +299,7 @@ pgd_t early_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
>> > >  static const pgprot_t protection_map[16] = {
>> > >  	[VM_NONE]					= PAGE_NONE,
>> > >  	[VM_READ]					= PAGE_READ,
>> > > -	[VM_WRITE]					= PAGE_COPY,
>> > > +	[VM_WRITE]					= PAGE_SHADOWSTACK,
>> > >  	[VM_WRITE | VM_READ]				= PAGE_COPY,
>> > >  	[VM_EXEC]					= PAGE_EXEC,
>> > >  	[VM_EXEC | VM_READ]				= PAGE_READ_EXEC,
>> > > diff --git a/mm/mmap.c b/mm/mmap.c
>> > > index d89770eaab6b..57a974f49b00 100644
>> > > --- a/mm/mmap.c
>> > > +++ b/mm/mmap.c
>> > > @@ -47,6 +47,7 @@
>> > >  #include <linux/oom.h>
>> > >  #include <linux/sched/mm.h>
>> > >  #include <linux/ksm.h>
>> > > +#include <linux/processor.h>
>> >
>> > It doesn't seem like this is necessary for this patch.
>>
>> Thanks. Yeah it looks like I forgot to remove this over the churn.
>> Will fix it.
>>
>> >
>> > - Charlie
>> >
>> > >
>> > >  #include <linux/uaccess.h>
>> > >  #include <asm/cacheflush.h>
>> > > --
>> > > 2.43.2
>> > >