The patch below does not apply to the 5.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.4.y git checkout FETCH_HEAD git cherry-pick -x d14fa1fcf69db9d070e75f1c4425211fa619dfc8 # <resolve conflicts, build, test, etc.> git commit -s git send-email --to 'stable@vger.kernel.org' --in-reply-to '2024040815-geometric-quaintly-f4a7@gregkh' --subject-prefix 'PATCH 5.4.y' HEAD^..

Possible dependencies:

d14fa1fcf69d ("riscv: process: Fix kernel gp leakage") fea2fed201ee ("riscv: Enable per-task stack canaries") d7071743db31 ("RISC-V: Add EFI stub support.") f2c9699f6555 ("riscv: Add STACKPROTECTOR supported") a5d8e55b2c7d ("Merge tag 'v5.7-rc7' into efi/core, to refresh the branch and pick up fixes")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From d14fa1fcf69db9d070e75f1c4425211fa619dfc8 Mon Sep 17 00:00:00 2001 From: Stefan O'Rear sorear@fastmail.com Date: Wed, 27 Mar 2024 02:12:58 -0400 Subject: [PATCH] riscv: process: Fix kernel gp leakage

childregs represents the registers which are active for the new thread in user context. For a kernel thread, childregs->gp is never used since the kernel gp is not touched by switch_to. For a user mode helper, the gp value can be observed in user space after execve or possibly by other means.

[From the email thread]

The /* Kernel thread */ comment is somewhat inaccurate in that it is also used for user_mode_helper threads, which exec a user process, e.g. /sbin/init or when /proc/sys/kernel/core_pattern is a pipe. Such threads do not have PF_KTHREAD set and are valid targets for ptrace etc. even before they exec.

childregs is the *user* context during syscall execution and it is observable from userspace in at least five ways:

1. kernel_execve does not currently clear integer registers, so the starting register state for PID 1 and other user processes started by the kernel has sp = user stack, gp = kernel __global_pointer$, all other integer registers zeroed by the memset in the patch comment.

This is a bug in its own right, but I'm unwilling to bet that it is the only way to exploit the issue addressed by this patch.

2. ptrace(PTRACE_GETREGSET): you can PTRACE_ATTACH to a user_mode_helper thread before it execs, but ptrace requires SIGSTOP to be delivered which can only happen at user/kernel boundaries.

3. /proc/*/task/*/syscall: this is perfectly happy to read pt_regs for user_mode_helpers before the exec completes, but gp is not one of the registers it returns.

4. PERF_SAMPLE_REGS_USER: LOCKDOWN_PERF normally prevents access to kernel addresses via PERF_SAMPLE_REGS_INTR, but due to this bug kernel addresses are also exposed via PERF_SAMPLE_REGS_USER which is permitted under LOCKDOWN_PERF. I have not attempted to write exploit code.

5. Much of the tracing infrastructure allows access to user registers. I have not attempted to determine which forms of tracing allow access to user registers without already allowing access to kernel registers.

Fixes: 7db91e57a0ac ("RISC-V: Task implementation") Cc: stable@vger.kernel.org Signed-off-by: Stefan O'Rear sorear@fastmail.com Reviewed-by: Alexandre Ghiti alexghiti@rivosinc.com Link: https://lore.kernel.org/r/20240327061258.2370291-1-sorear@fastmail.com Signed-off-by: Palmer Dabbelt palmer@rivosinc.com

diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c index 6abeecbfc51d..e4bc61c4e58a 100644 --- a/arch/riscv/kernel/process.c +++ b/arch/riscv/kernel/process.c @@ -27,8 +27,6 @@ #include <asm/vector.h> #include <asm/cpufeature.h>

-register unsigned long gp_in_global __asm__("gp"); - #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK) #include <linux/stackprotector.h> unsigned long __stack_chk_guard __read_mostly; @@ -207,7 +205,6 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args) if (unlikely(args->fn)) { /* Kernel thread */ memset(childregs, 0, sizeof(struct pt_regs)); - childregs->gp = gp_in_global; /* Supervisor/Machine, irqs on: */ childregs->status = SR_PP | SR_PIE;