Re: [PATCH v3 10/29] riscv/mm : ensure PROT_WRITE leads to VM_READ | VM_WRITE

Charlie Jenkins
2024-05-13T21:26:57Z

On Mon, May 13, 2024 at 11:41:34AM -0700, Deepak Gupta wrote:
> 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.

I would prefer the strict check. This is not critical though so I will
support whatever you decide!

- Charlie

>
> >
> > - 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
> > > > >