linux-kernel - Re: [PATCH 1/2] riscv: Use pointer masking to limit uaccess speculation

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Message-ID: <20251227212859.3a83d65e@pumpkin>
Date: Sat, 27 Dec 2025 21:28:59 +0000
From: David Laight <david.laight.linux@...il.com>
To: Deepak Gupta <debug@...osinc.com>
Cc: Lukas Gerlach <lukas.gerlach@...pa.de>, linux-riscv@...ts.infradead.org,
 palmer@...belt.com, pjw@...nel.org, aou@...s.berkeley.edu, alex@...ti.fr,
 linux-kernel@...r.kernel.org, daniel.weber@...pa.de,
 michael.schwarz@...pa.de, marton.bognar@...euven.be,
 jo.vanbulck@...euven.be
Subject: Re: [PATCH 1/2] riscv: Use pointer masking to limit uaccess
 speculation

On Fri, 19 Dec 2025 16:44:11 -0800
Deepak Gupta <debug@...osinc.com> wrote:

> On Thu, Dec 18, 2025 at 08:13:31PM +0100, Lukas Gerlach wrote:
> >Similarly to x86 and arm64, mitigate speculation past an access_ok()
> >check by masking the pointer before use.
> >
> >On RISC-V, user addresses have the MSB clear while kernel addresses
> >have the MSB set. The uaccess_mask_ptr() function clears the MSB,
> >ensuring any kernel pointer becomes invalid and will fault, while
> >valid user pointers remain unchanged. This prevents speculative
> >access to kernel memory via user copy functions.
> >
> >The masking is applied to __get_user, __put_user, raw_copy_from_user,
> >raw_copy_to_user, clear_user, and the unsafe_* variants.
> >
> >Signed-off-by: Lukas Gerlach <lukas.gerlach@...pa.de>
> >---
> > arch/riscv/include/asm/uaccess.h | 41 +++++++++++++++++++++++++-------
> > 1 file changed, 32 insertions(+), 9 deletions(-)
> >
> >diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
> >index 36bba6720c26..ceee1d62ff9b 100644
> >--- a/arch/riscv/include/asm/uaccess.h
> >+++ b/arch/riscv/include/asm/uaccess.h
> >@@ -74,6 +74,23 @@ static inline unsigned long __untagged_addr_remote(struct mm_struct *mm, unsigne
> > #define __typefits(x, type, not) \
> > 	__builtin_choose_expr(sizeof(x) <= sizeof(type), (unsigned type)0, not)
> >
> >+/*
> >+ * Sanitize a uaccess pointer such that it cannot reach any kernel address.
> >+ *
> >+ * On RISC-V, virtual addresses are sign-extended from the top implemented bit.
> >+ * User addresses have the MSB clear; kernel addresses have the MSB set.
> >+ * Clearing the MSB ensures any kernel pointer becomes non-canonical and will
> >+ * fault, while valid user pointers remain unchanged.
> >+ */
> >+#define uaccess_mask_ptr(ptr) ((__typeof__(ptr))__uaccess_mask_ptr(ptr))
> >+static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
> >+{
> >+	unsigned long val = (unsigned long)ptr;
> >+
> >+	val = (val << 1) >> 1;
> >+	return (void __user *)val;  
> 
> This is only clearing b63 which is what we don't need here.

It is also entirely the wrong operation.
A kernel address needs converting into an address that is guaranteed
to fault - not a user address that might be valid.

You need a guard page of address space between valid user addresses
and valid kernel addresses that is guaranteed to not be used.
Typically this is the top page of the user address space.
All kernel addresses need converting to the base of the guard page
using ALU operations (to avoid speculative accesses).
So: ptr = min(ptr, guard_page); easiest done (as in x86-64) with
a compare and conditional move.
The ppc version is more complex because there isn't a usable conditional
move instruction.
I think this would work for x86-32:
	offset = ptr + -guard_page;
	mask = sbb(x,x); // ~0u if the add set the carry flag.
	ptr -= offset & mask;
You need to find something that works for riscv.

> 
> You should be clearing b47 (if bit indexing starts at 0) on Sv48 and b56
> on Sv57 system.
> 
> Anything above b47/b56 isn't going to be used anyways in indexing into
> page tables and will be ignored if pointer masking is enabled at S.

Gah more broken hardware...
Ignoring the high address bits doesn't work and is really a bad idea.
Arm did it as well.
Trying to do 'user pointer masking' for uaccess validation is a PITA
if you have to allow for non-zero values in the high bits it makes
life even more complicated. 

And 'pointer masking' is so broken unless it is done at the page level
and enforced by the hardware.
What it does is make random/invalid addresses much more likely to be valid.
An interpreter can use them internally, but they are so slow software
masking  (following the validation) shouldn't be a real issue.

	David