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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: 7db91e57a0
("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>
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@ -27,8 +27,6 @@
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#include <asm/vector.h>
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#include <asm/cpufeature.h>
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register unsigned long gp_in_global __asm__("gp");
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#if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK)
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#include <linux/stackprotector.h>
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unsigned long __stack_chk_guard __read_mostly;
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@ -207,7 +205,6 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
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if (unlikely(args->fn)) {
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/* Kernel thread */
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memset(childregs, 0, sizeof(struct pt_regs));
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childregs->gp = gp_in_global;
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/* Supervisor/Machine, irqs on: */
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childregs->status = SR_PP | SR_PIE;
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