[PATCH] riscv, mm: Perform BPF exhandler fixup on page fault

[Björn Töpel]

From: Björn Töpel <bjorn at rivosinc.com>

Commit 21855cac82d3 ("riscv/mm: Prevent kernel module to access user
memory without uaccess routines") added early exits/deaths for page
faults stemming from accesses to user-space without using proper
uaccess routines (where sstatus.SUM is set).

Unfortunatly, this is too strict for some BPF programs, which relies
on BPF exhandler fixups. These BPF programs loads "BTF pointers". A
BTF pointers could either be a valid kernel pointer or NULL, but not a
userspace address.

Resolve the problem by calling the fixup handler in the early exit
path.

Fixes: 21855cac82d3 ("riscv/mm: Prevent kernel module to access user memory without uaccess routines")
Signed-off-by: Björn Töpel <bjorn at rivosinc.com>
---
Palmer,

This is a fix for BPF on riscv, but I'd still like to take it via the
RISC-V tree, given the mm changes.

BPF/BTF is a special snowflake, and needs special care. ;-)

If BPF_PROBE_MEM is ever to be used for usermode pointers in the
future, then the fixup call can be removed, in favor of setting
sstatus.SUM from the BPF jitted code.


Björn
---
 arch/riscv/mm/fault.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
index d86f7cebd4a7..eb0774d9c03b 100644
--- a/arch/riscv/mm/fault.c
+++ b/arch/riscv/mm/fault.c
@@ -267,10 +267,12 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
 	if (user_mode(regs))
 		flags |= FAULT_FLAG_USER;
 
-	if (!user_mode(regs) && addr < TASK_SIZE &&
-			unlikely(!(regs->status & SR_SUM)))
-		die_kernel_fault("access to user memory without uaccess routines",
-				addr, regs);
+	if (!user_mode(regs) && addr < TASK_SIZE && unlikely(!(regs->status & SR_SUM))) {
+		if (fixup_exception(regs))
+			return;
+
+		die_kernel_fault("access to user memory without uaccess routines", addr, regs);
+	}
 
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
 

base-commit: 950b879b7f0251317d26bae0687e72592d607532
-- 
2.37.2