[PATCH v2] riscv: futex: untag the user pointer before the atomic access

[Ziyi Guo <guoziyi114@gmail.com>]

access_ok() checks untagged_addr(uaddr), but arch_futex_atomic_op_inuser()
and futex_atomic_cmpxchg_inatomic() hand the raw uaddr to the inline asm
amoswap/amoadd/.../lr.w+sc.w through the "+m" (*uaddr) operand.  When the
tagged address ABI is enabled (CONFIG_RISCV_ISA_SUPM, prctl
PR_SET_TAGGED_ADDR_CTRL with PMLEN != 0), those two addresses differ: a
user pointer whose top PMLEN bits hold a tag passes access_ok() after
untagging, but the atomic is then performed on the still-tagged raw
address.

Supervisor-mode data accesses are not subject to the U-mode pointer
masking (that is governed by menvcfg.PMM, which the kernel does not set),
so hardware does not strip the tag for the kernel's own access.  With
Sv57 and PMLEN=16 the tag bits overlap the canonical-address bits, so a
tagged pointer can name a canonical kernel virtual address (e.g. in the
linear map) whose untagged form is a valid user address.  An unprivileged
process can thus make FUTEX_WAKE_OP perform an atomic read-modify-write on
an arbitrary kernel address, with the matching FUTEX_OP_CMP_* result
serving as a read oracle.

get_user()/put_user()/raw_copy_{to,from}_user() already untag the pointer
after the access_ok() check; do the same in the futex helpers so the
atomic operates on the address that was actually validated.

Fixes: 2e1743085887 ("riscv: Add support for the tagged address ABI")
Reviewed-by: Samuel Holland <samuel.holland@sifive.com>
Signed-off-by: Ziyi Guo <guoziyi114@gmail.com>
---
Changes in v2:
- Drop the inline comments (Samuel Holland)
- Add Samuel's Reviewed-by

v1: https://lore.kernel.org/all/20261001184355.2395068-1-guoziyi114@gmail.com/

 arch/riscv/include/asm/futex.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/riscv/include/asm/futex.h b/arch/riscv/include/asm/futex.h
index 90c86b115e00..6f3bc9bdb85e 100644
--- a/arch/riscv/include/asm/futex.h
+++ b/arch/riscv/include/asm/futex.h
@@ -40,6 +40,7 @@ arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
 
 	if (!access_ok(uaddr, sizeof(u32)))
 		return -EFAULT;
+	uaddr = untagged_addr(uaddr);
 
 	switch (op) {
 	case FUTEX_OP_SET:
@@ -82,6 +83,7 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
 
 	if (!access_ok(uaddr, sizeof(u32)))
 		return -EFAULT;
+	uaddr = untagged_addr(uaddr);
 
 	__enable_user_access();
 	__asm__ __volatile__ (
-- 
2.34.1


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