This is a note to let you know that I've just added the patch titled powerpc/mm: Fix lockup on kernel exec fault to the 5.12-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: powerpc-mm-fix-lockup-on-kernel-exec-fault.patch and it can be found in the queue-5.12 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From cd5d5e602f502895e47e18cd46804d6d7014e65c Mon Sep 17 00:00:00 2001 From: Christophe Leroy <christophe.leroy@xxxxxxxxxx> Date: Thu, 1 Jul 2021 11:17:08 +0000 Subject: powerpc/mm: Fix lockup on kernel exec fault From: Christophe Leroy <christophe.leroy@xxxxxxxxxx> commit cd5d5e602f502895e47e18cd46804d6d7014e65c upstream. The powerpc kernel is not prepared to handle exec faults from kernel. Especially, the function is_exec_fault() will return 'false' when an exec fault is taken by kernel, because the check is based on reading current->thread.regs->trap which contains the trap from user. For instance, when provoking a LKDTM EXEC_USERSPACE test, current->thread.regs->trap is set to SYSCALL trap (0xc00), and the fault taken by the kernel is not seen as an exec fault by set_access_flags_filter(). Commit d7df2443cd5f ("powerpc/mm: Fix spurious segfaults on radix with autonuma") made it clear and handled it properly. But later on commit d3ca587404b3 ("powerpc/mm: Fix reporting of kernel execute faults") removed that handling, introducing test based on error_code. And here is the problem, because on the 603 all upper bits of SRR1 get cleared when the TLB instruction miss handler bails out to ISI. Until commit cbd7e6ca0210 ("powerpc/fault: Avoid heavy search_exception_tables() verification"), an exec fault from kernel at a userspace address was indirectly caught by the lack of entry for that address in the exception tables. But after that commit the kernel mainly relies on KUAP or on core mm handling to catch wrong user accesses. Here the access is not wrong, so mm handles it. It is a minor fault because PAGE_EXEC is not set, set_access_flags_filter() should set PAGE_EXEC and voila. But as is_exec_fault() returns false as explained in the beginning, set_access_flags_filter() bails out without setting PAGE_EXEC flag, which leads to a forever minor exec fault. As the kernel is not prepared to handle such exec faults, the thing to do is to fire in bad_kernel_fault() for any exec fault taken by the kernel, as it was prior to commit d3ca587404b3. Fixes: d3ca587404b3 ("powerpc/mm: Fix reporting of kernel execute faults") Cc: stable@xxxxxxxxxxxxxxx # v4.14+ Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx> Acked-by: Nicholas Piggin <npiggin@xxxxxxxxx> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> Link: https://lore.kernel.org/r/024bb05105050f704743a0083fe3548702be5706.1625138205.git.christophe.leroy@xxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/powerpc/mm/fault.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-) --- a/arch/powerpc/mm/fault.c +++ b/arch/powerpc/mm/fault.c @@ -199,9 +199,7 @@ static bool bad_kernel_fault(struct pt_r { int is_exec = TRAP(regs) == 0x400; - /* NX faults set DSISR_PROTFAULT on the 8xx, DSISR_NOEXEC_OR_G on others */ - if (is_exec && (error_code & (DSISR_NOEXEC_OR_G | DSISR_KEYFAULT | - DSISR_PROTFAULT))) { + if (is_exec) { pr_crit_ratelimited("kernel tried to execute %s page (%lx) - exploit attempt? (uid: %d)\n", address >= TASK_SIZE ? "exec-protected" : "user", address, Patches currently in stable-queue which might be from christophe.leroy@xxxxxxxxxx are queue-5.12/mm-mremap-hold-the-rmap-lock-in-write-mode-when-moving-page-table-entries.patch queue-5.12/powerpc-mm-fix-lockup-on-kernel-exec-fault.patch