linux-kernel - [PATCHv3] x86/mm: set x32 syscall bit in SET

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Date:   Tue, 21 Mar 2017 20:47:11 +0300
From:   Dmitry Safonov <dsafonov@...tuozzo.com>
To:     <linux-kernel@...r.kernel.org>
CC:     <0x7f454c46@...il.com>, Dmitry Safonov <dsafonov@...tuozzo.com>,
        Adam Borowski <kilobyte@...band.pl>, <linux-mm@...ck.org>,
        Andrei Vagin <avagin@...il.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Borislav Petkov <bp@...e.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCHv3] x86/mm: set x32 syscall bit in SET_PERSONALITY()

After my changes to mmap(), its code now relies on the bitness of
performing syscall. According to that, it chooses the base of allocation:
mmap_base for 64-bit mmap() and mmap_compat_base for 32-bit syscall.
It was done by:
  commit 1b028f784e8c ("x86/mm: Introduce mmap_compat_base() for
32-bit mmap()").

The code afterwards relies on in_compat_syscall() returning true for
32-bit syscalls. It's usually so while we're in context of application
that does 32-bit syscalls. But during exec() it is not valid for x32 ELF.
The reason is that the application hasn't yet done any syscall, so x32
bit has not being set.
That results in -ENOMEM for x32 ELF files as there fired BAD_ADDR()
in elf_map(), that is called from do_execve()->load_elf_binary().
For i386 ELFs it works as SET_PERSONALITY() sets TS_COMPAT flag.

Set x32 bit before first return to userspace, during setting personality
at exec(). This way we can rely on in_compat_syscall() during exec().
Do also the reverse: drop x32 syscall bit at SET_PERSONALITY for 64-bits.

Fixes: commit 1b028f784e8c ("x86/mm: Introduce mmap_compat_base() for
32-bit mmap()")
Cc: 0x7f454c46@...il.com
Cc: linux-mm@...ck.org
Cc: Andrei Vagin <avagin@...il.com>
Cc: Cyrill Gorcunov <gorcunov@...nvz.org>
Cc: Borislav Petkov <bp@...e.de>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: x86@...nel.org
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Reported-by: Adam Borowski <kilobyte@...band.pl>
Signed-off-by: Dmitry Safonov <dsafonov@...tuozzo.com>
---
v2:
- specifying mmap() allocation path which failed during exec()
- fix comment style
v3:
- clear x32 syscall flag during x32 -> x86-64 exec() (thanks, HPA).

 arch/x86/kernel/process_64.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index d6b784a5520d..b03f186369eb 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -493,6 +493,8 @@ void set_personality_64bit(void)
 	clear_thread_flag(TIF_IA32);
 	clear_thread_flag(TIF_ADDR32);
 	clear_thread_flag(TIF_X32);
+	/* Drop x32 syscall bit, so in_compat_syscall() will return false. */
+	task_pt_regs(current)->orig_ax &= ~__X32_SYSCALL_BIT;
 
 	/* Ensure the corresponding mm is not marked. */
 	if (current->mm)
@@ -519,8 +521,14 @@ void set_personality_ia32(bool 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 */
+		/*
+		 * in_compat_syscall() uses the presence of the x32
+		 * syscall bit flag to determine compat status.
+		 * On the bitness of syscall relies x86 mmap() code,
+		 * so set x32 syscall bit right here to make
+		 * in_compat_syscall() work during exec().
+		 */
+		task_pt_regs(current)->orig_ax |= __X32_SYSCALL_BIT;
 		current->thread.status &= ~TS_COMPAT;
 	} else {
 		set_thread_flag(TIF_IA32);
-- 
2.12.0