[RFC RESEND v2 02/13] powerpc: mm: Fix kfence page fault reporting

["Ritesh Harjani (IBM)" <ritesh.list@gmail.com>]

copy_from_kernel_nofault() can be called when doing read of /proc/kcore.
/proc/kcore can have some unmapped kfence objects which when read via
copy_from_kernel_nofault() can cause page faults. Since *_nofault()
functions define their own fixup table for handling fault, use that
instead of asking kfence to handle such faults.

Hence we search the exception tables for the nip which generated the
fault. If there is an entry then we let the fixup table handler handle the
page fault by returning an error from within ___do_page_fault().

This can be easily triggered if someone tries to do dd from /proc/kcore.
dd if=/proc/kcore of=/dev/null bs=1M

<some example false negatives>
===============================
BUG: KFENCE: invalid read in copy_from_kernel_nofault+0xb0/0x1c8
Invalid read at 0x000000004f749d2e:
 copy_from_kernel_nofault+0xb0/0x1c8
 0xc0000000057f7950
 read_kcore_iter+0x41c/0x9ac
 proc_reg_read_iter+0xe4/0x16c
 vfs_read+0x2e4/0x3b0
 ksys_read+0x88/0x154
 system_call_exception+0x124/0x340
 system_call_common+0x160/0x2c4

BUG: KFENCE: use-after-free read in copy_from_kernel_nofault+0xb0/0x1c8
Use-after-free read at 0x000000008fbb08ad (in kfence-#0):
 copy_from_kernel_nofault+0xb0/0x1c8
 0xc0000000057f7950
 read_kcore_iter+0x41c/0x9ac
 proc_reg_read_iter+0xe4/0x16c
 vfs_read+0x2e4/0x3b0
 ksys_read+0x88/0x154
 system_call_exception+0x124/0x340
 system_call_common+0x160/0x2c4

Guessing the fix should go back to when we first got kfence on PPC32.

Fixes: 90cbac0e995d ("powerpc: Enable KFENCE for PPC32")
Reported-by: Disha Goel <disgoel@linux.ibm.com>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
 arch/powerpc/mm/fault.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index 81c77ddce2e3..fa825198f29f 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -439,9 +439,17 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
 	/*
 	 * The kernel should never take an execute fault nor should it
 	 * take a page fault to a kernel address or a page fault to a user
-	 * address outside of dedicated places
+	 * address outside of dedicated places.
+	 *
+	 * Rather than kfence reporting false negatives, let the fixup table
+	 * handler handle the page fault by returning SIGSEGV, if the fault
+	 * has come from functions like copy_from_kernel_nofault().
 	 */
 	if (unlikely(!is_user && bad_kernel_fault(regs, error_code, address, is_write))) {
+
+		if (search_exception_tables(instruction_pointer(regs)))
+			return SIGSEGV;
+
 		if (kfence_handle_page_fault(address, is_write, regs))
 			return 0;
 
-- 
2.46.0