x86/entry: Unbreak 32bit fast syscall

Andy reported that the syscall treacing for 32bit fast syscall fails:

# ./tools/testing/selftests/x86/ptrace_syscall_32
...
[RUN] SYSEMU
[FAIL] Initial args are wrong (nr=224, args=10 11 12 13 14 4289172732)
...
[RUN] SYSCALL
[FAIL] Initial args are wrong (nr=29, args=0 0 0 0 0 4289172732)
 
The eason is that the conversion to generic entry code moved the retrieval
of the sixth argument (EBP) after the point where the syscall entry work
runs, i.e. ptrace, seccomp, audit...

Unbreak it by providing a split up version of syscall_enter_from_user_mode().

- syscall_enter_from_user_mode_prepare() establishes state and enables
  interrupts

- syscall_enter_from_user_mode_work() runs the entry work

Replace the call to syscall_enter_from_user_mode() in the 32bit fast
syscall C-entry with the split functions and stick the EBP retrieval
between them.

Fixes: 27d6b4d14f5c ("x86/entry: Use generic syscall entry function")
Reported-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/87k0xdjbtt.fsf@nanos.tec.linutronix.de

1 files changed, 42 insertions, 9 deletions

diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index efebbffcd5cc..159c7476b11b 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -110,15 +110,30 @@ static inline __must_check int arch_syscall_enter_tracehook(struct pt_regs *regs
 #endif
 
 /**
- * syscall_enter_from_user_mode - Check and handle work before invoking
- *				 a syscall
+ * syscall_enter_from_user_mode_prepare - Establish state and enable interrupts
  * @regs:	Pointer to currents pt_regs
- * @syscall:	The syscall number
  *
  * Invoked from architecture specific syscall entry code with interrupts
  * disabled. The calling code has to be non-instrumentable. When the
- * function returns all state is correct and the subsequent functions can be
- * instrumented.
+ * function returns all state is correct, interrupts are enabled and the
+ * subsequent functions can be instrumented.
+ *
+ * This handles lockdep, RCU (context tracking) and tracing state.
+ *
+ * This is invoked when there is extra architecture specific functionality
+ * to be done between establishing state and handling user mode entry work.
+ */
+void syscall_enter_from_user_mode_prepare(struct pt_regs *regs);
+
+/**
+ * syscall_enter_from_user_mode_work - Check and handle work before invoking
+ *				       a syscall
+ * @regs:	Pointer to currents pt_regs
+ * @syscall:	The syscall number
+ *
+ * Invoked from architecture specific syscall entry code with interrupts
+ * enabled after invoking syscall_enter_from_user_mode_prepare() and extra
+ * architecture specific work.
  *
  * Returns: The original or a modified syscall number
  *
@@ -127,12 +142,30 @@ static inline __must_check int arch_syscall_enter_tracehook(struct pt_regs *regs
  * syscall_set_return_value() first.  If neither of those are called and -1
  * is returned, then the syscall will fail with ENOSYS.
  *
- * The following functionality is handled here:
+ * It handles the following work items:
  *
- *  1) Establish state (lockdep, RCU (context tracking), tracing)
- *  2) TIF flag dependent invocations of arch_syscall_enter_tracehook(),
+ *  1) TIF flag dependent invocations of arch_syscall_enter_tracehook(),
  *     __secure_computing(), trace_sys_enter()
- *  3) Invocation of audit_syscall_entry()
+ *  2) Invocation of audit_syscall_entry()
+ */
+long syscall_enter_from_user_mode_work(struct pt_regs *regs, long syscall);
+
+/**
+ * syscall_enter_from_user_mode - Establish state and check and handle work
+ *				  before invoking a syscall
+ * @regs:	Pointer to currents pt_regs
+ * @syscall:	The syscall number
+ *
+ * Invoked from architecture specific syscall entry code with interrupts
+ * disabled. The calling code has to be non-instrumentable. When the
+ * function returns all state is correct, interrupts are enabled and the
+ * subsequent functions can be instrumented.
+ *
+ * This is combination of syscall_enter_from_user_mode_prepare() and
+ * syscall_enter_from_user_mode_work().
+ *
+ * Returns: The original or a modified syscall number. See
+ * syscall_enter_from_user_mode_work() for further explanation.
  */
 long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall);