[RFC] x86/vdso: Align vdso after searching for free area

[Dmitry Safonov <dima@arista.com>]

There is errata for AMD family 15h CPUs [1] and since
commit dfb09f9b7ab03 ("x86, amd: Avoid cache aliasing penalties on AMD
family 15h") bits [14:12] are being cleared for shared libraries.
Also per-boot ASLR applies over upper bits by OR directly over the
address.

As we need special alignment and lower bits values to be set, it makes
only a little sense to call get_unmapped_area() after calculating the
address. It also can lead to random crashes if get_unmapped_area()
actually changes/aligns the address, which we observed on 15h CPU.
Usually it's not a problem as there isn't many mappings (except possibly
ld.so, uprobes?) and result address is the same before/after
get_unmapped_area().

Move align_vdso_addr() after get_unmapped_area() to make sure that
errata for AMD 15h is always applied.

[1]: https://developer.amd.com/wordpress/media/2012/10/SharedL1InstructionCacheonAMD15hCPU.pdf
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vasiliy Khoruzhick <vasilykh@arista.com>
Cc: x86@kernel.org
Signed-off-by: Dmitry Safonov <dima@arista.com>
---
 arch/x86/entry/vdso/vma.c    | 18 ++++++++++--------
 arch/x86/include/asm/elf.h   |  1 +
 arch/x86/kernel/sys_x86_64.c |  2 +-
 3 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c
index 5b8b556dbb12..862f0cce3ee6 100644
--- a/arch/x86/entry/vdso/vma.c
+++ b/arch/x86/entry/vdso/vma.c
@@ -154,18 +154,26 @@ static int map_vdso(const struct vdso_image *image, unsigned long addr)
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma;
 	unsigned long text_start;
+	unsigned long area_size = image->size - image->sym_vvar_start;
 	int ret = 0;
 
 	if (down_write_killable(&mm->mmap_sem))
 		return -EINTR;
 
-	addr = get_unmapped_area(NULL, addr,
-				 image->size - image->sym_vvar_start, 0, 0);
+	/* Find a bigger place for vma then needed - to align vdso later. */
+	area_size += get_align_mask();
+	addr = get_unmapped_area(NULL, addr, area_size, 0, 0);
 	if (IS_ERR_VALUE(addr)) {
 		ret = addr;
 		goto up_fail;
 	}
 
+	/*
+	 * Forcibly align the final address in case we have a hardware
+	 * issue that requires alignment for performance reasons.
+	 */
+	addr = align_vdso_addr(addr);
+
 	text_start = addr - image->sym_vvar_start;
 
 	/*
@@ -239,12 +247,6 @@ static unsigned long vdso_addr(unsigned long start, unsigned len)
 		addr = start;
 	}
 
-	/*
-	 * Forcibly align the final address in case we have a hardware
-	 * issue that requires alignment for performance reasons.
-	 */
-	addr = align_vdso_addr(addr);
-
 	return addr;
 }
 
diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 0d157d2a1e2a..88aa49294c9c 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -382,4 +382,5 @@ struct va_alignment {
 
 extern struct va_alignment va_align;
 extern unsigned long align_vdso_addr(unsigned long);
+extern unsigned long get_align_mask(void);
 #endif /* _ASM_X86_ELF_H */
diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c
index 6a78d4b36a79..4dd74c6f236d 100644
--- a/arch/x86/kernel/sys_x86_64.c
+++ b/arch/x86/kernel/sys_x86_64.c
@@ -27,7 +27,7 @@
 /*
  * Align a virtual address to avoid aliasing in the I$ on AMD F15h.
  */
-static unsigned long get_align_mask(void)
+unsigned long get_align_mask(void)
 {
 	/* handle 32- and 64-bit case with a single conditional */
 	if (va_align.flags < 0 || !(va_align.flags & (2 - mmap_is_ia32())))
-- 
2.13.6