6.6-stable review patch. If anyone has any objections, please let me know.
------------------
From: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
[ Upstream commit 06dbbb4d5f7126b6307ab807cbf04ecfc459b933 ]
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.
eg. dd if=/proc/kcore of=/dev/null bs=1M
Some example false negatives:
===============================
BUG: KFENCE: invalid read in copy_from_kernel_nofault+0x9c/0x1a0
Invalid read at 0xc0000000fdff0000:
copy_from_kernel_nofault+0x9c/0x1a0
0xc00000000665f950
read_kcore_iter+0x57c/0xa04
proc_reg_read_iter+0xe4/0x16c
vfs_read+0x320/0x3ec
ksys_read+0x90/0x154
system_call_exception+0x120/0x310
system_call_vectored_common+0x15c/0x2ec
BUG: KFENCE: use-after-free read in copy_from_kernel_nofault+0x9c/0x1a0
Use-after-free read at 0xc0000000fe050000 (in kfence-#2):
copy_from_kernel_nofault+0x9c/0x1a0
0xc00000000665f950
read_kcore_iter+0x57c/0xa04
proc_reg_read_iter+0xe4/0x16c
vfs_read+0x320/0x3ec
ksys_read+0x90/0x154
system_call_exception+0x120/0x310
system_call_vectored_common+0x15c/0x2ec
Fixes: 90cbac0e995d ("powerpc: Enable KFENCE for PPC32")
Suggested-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
Reported-by: Disha Goel <disgoel@xxxxxxxxxxxxx>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
Reviewed-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Link: https://patch.msgid.link/a411788081d50e3b136c6270471e35aba3dfafa3.1729271995.git.ritesh.list@xxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
arch/powerpc/mm/fault.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index b1723094d464c..d3e0f5b3ecc74 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -431,10 +431,16 @@ 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 directly reporting false negatives, search whether
+ * the NIP belongs to the fixup table for cases where fault could come
+ * from functions like copy_from_kernel_nofault().
*/
if (unlikely(!is_user && bad_kernel_fault(regs, error_code, address, is_write))) {
- if (kfence_handle_page_fault(address, is_write, regs))
+ if (is_kfence_address((void *)address) &&
+ !search_exception_tables(instruction_pointer(regs)) &&
+ kfence_handle_page_fault(address, is_write, regs))
return 0;
return SIGSEGV;
--
2.43.0
