[PATCH 6.1 124/138] riscv: process: Fix kernel gp leakage

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6.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Stefan O'Rear <sorear@xxxxxxxxxxxx>

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@xxxxxxxxxxxxxxx
Signed-off-by: Stefan O'Rear <sorear@xxxxxxxxxxxx>
Reviewed-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20240327061258.2370291-1-sorear@xxxxxxxxxxxx
Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/riscv/kernel/process.c |    3 ---
 1 file changed, 3 deletions(-)

--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -25,8 +25,6 @@
 #include <asm/thread_info.h>
 #include <asm/cpuidle.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;
@@ -170,7 +168,6 @@ int copy_thread(struct task_struct *p, c
 	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;
 






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