x86/mm: Introduce mmap_compat_base() for 32-bit mmap()

mmap() uses a base address, from which it starts to look for a free space
for allocation.

The base address is stored in mm->mmap_base, which is calculated during
exec(). The address depends on task's size, set rlimit for stack, ASLR
randomization. The base depends on the task size and the number of random
bits which are different for 64-bit and 32bit applications.

Due to the fact, that the base address is fixed, its mmap() from a compat
(32bit) syscall issued by a 64bit task will return a address which is based
on the 64bit base address and does not fit into the 32bit address space
(4GB). The returned pointer is truncated to 32bit, which results in an
invalid address.

To solve store a seperate compat address base plus a compat legacy address
base in mm_struct. These bases are calculated at exec() time and can be
used later to address the 32bit compat mmap() issued by 64 bit
applications.

As a consequence of this change 32-bit applications issuing a 64-bit
syscall (after doing a long jump) will get a 64-bit mapping now. Before
this change 32-bit applications always got a 32bit mapping.

[ tglx: Massaged changelog and added a comment ]

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Cc: 0x7f454c46@gmail.com
Cc: linux-mm@kvack.org
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Link: http://lkml.kernel.org/r/20170306141721.9188-4-dsafonov@virtuozzo.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 34 insertions, 13 deletions

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 1e9cb945dca1..529ab79800af 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -36,11 +36,16 @@ struct va_alignment __read_mostly va_align = {
 	.flags = -1,
 };
 
-static inline unsigned long tasksize_32bit(void)
+unsigned long tasksize_32bit(void)
 {
 	return IA32_PAGE_OFFSET;
 }
 
+unsigned long tasksize_64bit(void)
+{
+	return TASK_SIZE_MAX;
+}
+
 static unsigned long stack_maxrandom_size(unsigned long task_size)
 {
 	unsigned long max = 0;
@@ -81,6 +86,8 @@ static unsigned long arch_rnd(unsigned int rndbits)
 
 unsigned long arch_mmap_rnd(void)
 {
+	if (!(current->flags & PF_RANDOMIZE))
+		return 0;
 	return arch_rnd(mmap_is_ia32() ? mmap32_rnd_bits : mmap64_rnd_bits);
 }
 
@@ -114,22 +121,36 @@ static unsigned long mmap_legacy_base(unsigned long rnd,
  * This function, called very early during the creation of a new
  * process VM image, sets up which VM layout function to use:
  */
-void arch_pick_mmap_layout(struct mm_struct *mm)
+static void arch_pick_mmap_base(unsigned long *base, unsigned long *legacy_base,
+		unsigned long random_factor, unsigned long task_size)
 {
-	unsigned long random_factor = 0UL;
-
-	if (current->flags & PF_RANDOMIZE)
-		random_factor = arch_mmap_rnd();
-
-	mm->mmap_legacy_base = mmap_legacy_base(random_factor, TASK_SIZE);
+	*legacy_base = mmap_legacy_base(random_factor, task_size);
+	if (mmap_is_legacy())
+		*base = *legacy_base;
+	else
+		*base = mmap_base(random_factor, task_size);
+}
 
-	if (mmap_is_legacy()) {
-		mm->mmap_base = mm->mmap_legacy_base;
+void arch_pick_mmap_layout(struct mm_struct *mm)
+{
+	if (mmap_is_legacy())
 		mm->get_unmapped_area = arch_get_unmapped_area;
-	} else {
-		mm->mmap_base = mmap_base(random_factor, TASK_SIZE);
+	else
 		mm->get_unmapped_area = arch_get_unmapped_area_topdown;
-	}
+
+	arch_pick_mmap_base(&mm->mmap_base, &mm->mmap_legacy_base,
+			arch_rnd(mmap64_rnd_bits), tasksize_64bit());
+
+#ifdef CONFIG_HAVE_ARCH_COMPAT_MMAP_BASES
+	/*
+	 * The mmap syscall mapping base decision depends solely on the
+	 * syscall type (64-bit or compat). This applies for 64bit
+	 * applications and 32bit applications. The 64bit syscall uses
+	 * mmap_base, the compat syscall uses mmap_compat_base.
+	 */
+	arch_pick_mmap_base(&mm->mmap_compat_base, &mm->mmap_compat_legacy_base,
+			arch_rnd(mmap32_rnd_bits), tasksize_32bit());
+#endif
 }
 
 const char *arch_vma_name(struct vm_area_struct *vma)