5.15-stable review patch. If anyone has any objections, please let me know.
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From: Stefan O'Rear sorear@fastmail.com
commit d14fa1fcf69db9d070e75f1c4425211fa619dfc8 upstream.
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/riscv/kernel/process.c | 3 --- 1 file changed, 3 deletions(-)
--- a/arch/riscv/kernel/process.c +++ b/arch/riscv/kernel/process.c @@ -24,8 +24,6 @@ #include <asm/switch_to.h> #include <asm/thread_info.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; @@ -130,7 +128,6 @@ int copy_thread(unsigned long clone_flag if (unlikely(p->flags & (PF_KTHREAD | PF_IO_WORKER))) { /* Kernel thread */ memset(childregs, 0, sizeof(struct pt_regs)); - childregs->gp = gp_in_global; /* Supervisor/Machine, irqs on: */ childregs->status = SR_PP | SR_PIE;