[Devel] [PATCH 11/11] x86/mm: Make in_compat_syscall() work during exec

[Dmitry Safonov]

The x86 mmap() code selects the mmap base for an allocation depending on
the bitness of the syscall. For 64bit sycalls it select mm->mmap_base and
for 32bit mm->mmap_compat_base.

On execve the registers of the task invoking exec() are copied to the child
pt_regs. So child->pt_regs->orig_ax contains the execve syscall number of the
parent.

exec() calls mmap() which in turn uses in_compat_syscall() to check whether
the mapping is for a 32bit or a 64bit task. The decision is made on the
following criteria:

  ia32	  child->thread.status & TS_COMPAT
   x32	  child->pt_regs.orig_ax & __X32_SYSCALL_BIT
  ia64	  !ia32 && !x32

child->thread.status is corretly set up in set_personality_*(), but the
syscall number in child->pt_regs.orig_ax is left unmodified.

Therefore the parent/child combinations work or fail in the following way:

Parent Child Child->thread_status  child->pt_regs.orig_ax  in_compat()  Works
ia64    ia64   TS_COMPAT == 0	   __X32_SYSCALL_BIT == 0     false       Y
ia64    ia32   TS_COMPAT == 1	   __X32_SYSCALL_BIT == 0     true        Y
ia64     x32   TS_COMPAT == 0	   __X32_SYSCALL_BIT == 0     false       N
ia32    ia64   TS_COMPAT == 0	   __X32_SYSCALL_BIT == 0     false       Y
ia32    ia32   TS_COMPAT == 1	   __X32_SYSCALL_BIT == 0     true        Y
ia32     x32   TS_COMPAT == 0	   __X32_SYSCALL_BIT == 0     false       N
 x32    ia64   TS_COMPAT == 0	   __X32_SYSCALL_BIT == 1     true        N
 x32    ia32   TS_COMPAT == 1	   __X32_SYSCALL_BIT == 1     true        Y
 x32     x32   TS_COMPAT == 0	   __X32_SYSCALL_BIT == 1     true        Y

Make set_personality_*() store the syscall number incl. __X32_SYSCALL_BIT
which corresponds to the newly started ELF executable in the childs
pt_regs, i.e. pretend that the exec was invoked from a task with the same
executable format.

So both thread.status and pt_regs.orig_ax correspond to the new ELF format
and in_compat_syscall() returns the correct result.

[ tglx: Rewrote changelog ]

Fixes: commit 1b028f784e8c ("x86/mm: Introduce mmap_compat_base() for 32-bit mmap()")
Reported-by: Adam Borowski <kilobyte at angband.pl>
Suggested-by: H. Peter Anvin <hpa at zytor.com>
Suggested-by: Thomas Gleixner <tglx at linutronix.de>
Signed-off-by: Dmitry Safonov <dsafonov at virtuozzo.com>
Cc: 0x7f454c46 at gmail.com
Cc: linux-mm at kvack.org
Cc: Andrei Vagin <avagin at gmail.com>
Cc: Andy Lutomirski <luto at kernel.org>
Cc: Cyrill Gorcunov <gorcunov at openvz.org>
Cc: Borislav Petkov <bp at suse.de>
Cc: "Kirill A. Shutemov" <kirill.shutemov at linux.intel.com>
Link: http://lkml.kernel.org/r/20170331111137.28170-1-dsafonov@virtuozzo.com
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>

[peeked from ms commit ada26481dfe6]
Signed-off-by: Dmitry Safonov <dsafonov at virtuozzo.com>
---
 arch/x86/kernel/process_64.c | 67 ++++++++++++++++++++++++++++++--------------
 1 file changed, 46 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 2485430f4f2a..d2e444cb7209 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -49,6 +49,11 @@
 #include <asm/syscalls.h>
 #include <asm/debugreg.h>
 #include <asm/switch_to.h>
+#include <asm/unistd.h>
+#ifdef CONFIG_IA32_EMULATION
+/* Not included via unistd.h */
+#include <asm/unistd_32_ia32.h>
+#endif
 
 asmlinkage extern void ret_from_fork(void);
 
@@ -426,6 +431,8 @@ void set_personality_64bit(void)
 	clear_thread_flag(TIF_IA32);
 	clear_thread_flag(TIF_ADDR32);
 	clear_thread_flag(TIF_X32);
+	/* Pretend that this comes from a 64bit execve */
+	task_pt_regs(current)->orig_ax = __NR_execve;
 
 	/* Ensure the corresponding mm is not marked. */
 	if (current->mm)
@@ -438,32 +445,50 @@ void set_personality_64bit(void)
 	current->personality &= ~READ_IMPLIES_EXEC;
 }
 
-void set_personality_ia32(bool x32)
+static void __set_personality_x32(void)
 {
-	/* inherit personality from parent */
+#ifdef CONFIG_X86_X32
+	clear_thread_flag(TIF_IA32);
+	set_thread_flag(TIF_X32);
+	if (current->mm)
+		current->mm->context.ia32_compat = TIF_X32;
+	current->personality &= ~READ_IMPLIES_EXEC;
+	/*
+	 * in_compat_syscall() uses the presence of the x32 syscall bit
+	 * flag to determine compat status.  The x86 mmap() code relies on
+	 * the syscall bitness so set x32 syscall bit right here to make
+	 * in_compat_syscall() work during exec().
+	 *
+	 * Pretend to come from a x32 execve.
+	 */
+	task_pt_regs(current)->orig_ax = __NR_x32_execve | __X32_SYSCALL_BIT;
+	current_thread_info()->status &= ~TS_COMPAT;
+#endif
+}
 
+static void __set_personality_ia32(void)
+{
+#ifdef CONFIG_IA32_EMULATION
+	set_thread_flag(TIF_IA32);
+	clear_thread_flag(TIF_X32);
+	if (current->mm)
+		current->mm->context.ia32_compat = TIF_IA32;
+	current->personality |= force_personality32;
+	/* Prepare the first "return" to user space */
+	task_pt_regs(current)->orig_ax = __NR_ia32_execve;
+	current_thread_info()->status |= TS_COMPAT;
+#endif
+}
+
+void set_personality_ia32(bool x32)
+{
 	/* Make sure to be in 32bit mode */
 	set_thread_flag(TIF_ADDR32);
 
-	/* Mark the associated mm as containing 32-bit tasks. */
-	if (x32) {
-		clear_thread_flag(TIF_IA32);
-		set_thread_flag(TIF_X32);
-		if (current->mm)
-			current->mm->context.ia32_compat = TIF_X32;
-		current->personality &= ~READ_IMPLIES_EXEC;
-		/* is_compat_task() uses the presence of the x32
-		   syscall bit flag to determine compat status */
-		current_thread_info()->status &= ~TS_COMPAT;
-	} else {
-		set_thread_flag(TIF_IA32);
-		clear_thread_flag(TIF_X32);
-		if (current->mm)
-			current->mm->context.ia32_compat = TIF_IA32;
-		current->personality |= force_personality32;
-		/* Prepare the first "return" to user space */
-		current_thread_info()->status |= TS_COMPAT;
-	}
+	if (x32)
+		__set_personality_x32();
+	else
+		__set_personality_ia32();
 }
 EXPORT_SYMBOL_GPL(set_personality_ia32);
 
-- 
2.12.2