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Message-Id: <9268050ac1fb3db6b4ec20d3ef696cc44fa3e9d0.1611884439.git.luto@kernel.org>
Date: Thu, 28 Jan 2021 17:41:21 -0800
From: Andy Lutomirski <luto@...nel.org>
To: x86@...nel.org
Cc: LKML <linux-kernel@...r.kernel.org>,
Andy Lutomirski <luto@...nel.org>
Subject: [PATCH] x86/ptrace: Clean up PTRACE_GETREGS/PTRACE_PUTREGS regset selection
task_user_regset_view() is fundamentally broken, but it's ABI for
PTRACE_GETREGSET and PTRACE_SETREGSET.
We shouldn't be using it for PTRACE_GETREGS or PTRACE_SETREGS,
though. A native 64-bit ptrace() call and an x32 ptrace() call
should use the 64-bit regset views, and a 32-bit ptrace() call
(native or compat) should use the 32-bit regset.
task_user_regset_view() almost does this except that it will
malfunction if a ptracer is itself ptraced and the outer ptracer
modifies CS on entry to a ptrace() syscall. Hopefully that has
never happened. (The compat ptrace() code already hardcoded the
32-bit regset, so this patch has no effect on that path.)
Fix it and deobfuscate the code by hardcoding the 64-bit view in the
x32 ptrace() and selecting the view based on the kernel config in
the native ptrace().
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
Every time I look at ptrace, it grosses me out. This makes it slightly
more comprehensible.
arch/x86/kernel/ptrace.c | 37 +++++++++++++++++++++++++++++--------
1 file changed, 29 insertions(+), 8 deletions(-)
diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index bedca011459c..ed8f153cd302 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -704,6 +704,9 @@ void ptrace_disable(struct task_struct *child)
#if defined CONFIG_X86_32 || defined CONFIG_IA32_EMULATION
static const struct user_regset_view user_x86_32_view; /* Initialized below. */
#endif
+#ifdef CONFIG_X86_64
+static const struct user_regset_view user_x86_64_view; /* Initialized below. */
+#endif
long arch_ptrace(struct task_struct *child, long request,
unsigned long addr, unsigned long data)
@@ -711,6 +714,14 @@ long arch_ptrace(struct task_struct *child, long request,
int ret;
unsigned long __user *datap = (unsigned long __user *)data;
+#ifdef CONFIG_X86_64
+ /* This is native 64-bit ptrace() */
+ const struct user_regset_view *regset_view = &user_x86_64_view;
+#else
+ /* This is native 32-bit ptrace() */
+ const struct user_regset_view *regset_view = &user_x86_32_view;
+#endif
+
switch (request) {
/* read the word at location addr in the USER area. */
case PTRACE_PEEKUSR: {
@@ -749,28 +760,28 @@ long arch_ptrace(struct task_struct *child, long request,
case PTRACE_GETREGS: /* Get all gp regs from the child. */
return copy_regset_to_user(child,
- task_user_regset_view(current),
+ regset_view,
REGSET_GENERAL,
0, sizeof(struct user_regs_struct),
datap);
case PTRACE_SETREGS: /* Set all gp regs in the child. */
return copy_regset_from_user(child,
- task_user_regset_view(current),
+ regset_view,
REGSET_GENERAL,
0, sizeof(struct user_regs_struct),
datap);
case PTRACE_GETFPREGS: /* Get the child FPU state. */
return copy_regset_to_user(child,
- task_user_regset_view(current),
+ regset_view,
REGSET_FP,
0, sizeof(struct user_i387_struct),
datap);
case PTRACE_SETFPREGS: /* Set the child FPU state. */
return copy_regset_from_user(child,
- task_user_regset_view(current),
+ regset_view,
REGSET_FP,
0, sizeof(struct user_i387_struct),
datap);
@@ -1152,28 +1163,28 @@ static long x32_arch_ptrace(struct task_struct *child,
case PTRACE_GETREGS: /* Get all gp regs from the child. */
return copy_regset_to_user(child,
- task_user_regset_view(current),
+ &user_x86_64_view,
REGSET_GENERAL,
0, sizeof(struct user_regs_struct),
datap);
case PTRACE_SETREGS: /* Set all gp regs in the child. */
return copy_regset_from_user(child,
- task_user_regset_view(current),
+ &user_x86_64_view,
REGSET_GENERAL,
0, sizeof(struct user_regs_struct),
datap);
case PTRACE_GETFPREGS: /* Get the child FPU state. */
return copy_regset_to_user(child,
- task_user_regset_view(current),
+ &user_x86_64_view,
REGSET_FP,
0, sizeof(struct user_i387_struct),
datap);
case PTRACE_SETFPREGS: /* Set the child FPU state. */
return copy_regset_from_user(child,
- task_user_regset_view(current),
+ &user_x86_64_view,
REGSET_FP,
0, sizeof(struct user_i387_struct),
datap);
@@ -1309,6 +1320,16 @@ void __init update_regset_xstate_info(unsigned int size, u64 xstate_mask)
xstate_fx_sw_bytes[USER_XSTATE_XCR0_WORD] = xstate_mask;
}
+/*
+ * This is used by PTRACE_GETREGSET and PTRACE_SETREGSET to decide which
+ * regset format to use based on the register state of the tracee.
+ * This makes no sense whatsoever, but there appears to be existing user
+ * code that relies on it.
+ *
+ * The best way to fix it in the long run would probably be to add
+ * new improved ptrace() APIs to read and write registers reliably.
+ * Good luck.
+ */
const struct user_regset_view *task_user_regset_view(struct task_struct *task)
{
#ifdef CONFIG_IA32_EMULATION
--
2.29.2
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