thp: add thp_get_unmapped_area_vmflags()

When memory is being placed, mmap() will take care to respect the guard
gaps of certain types of memory (VM_SHADOWSTACK, VM_GROWSUP and
VM_GROWSDOWN).  In order to ensure guard gaps between mappings, mmap()
needs to consider two things:

 1. That the new mapping isn't placed in an any existing mappings guard
    gaps.
 2. That the new mapping isn't placed such that any existing mappings
    are not in *its* guard gaps.

The longstanding behavior of mmap() is to ensure 1, but not take any care
around 2.  So for example, if there is a PAGE_SIZE free area, and a mmap()
with a PAGE_SIZE size, and a type that has a guard gap is being placed,
mmap() may place the shadow stack in the PAGE_SIZE free area.  Then the
mapping that is supposed to have a guard gap will not have a gap to the
adjacent VMA.

Add a THP implementations of the vm_flags variant of get_unmapped_area(). 
Future changes will call this from mmap.c in the do_mmap() path to allow
shadow stacks to be placed with consideration taken for the start guard
gap.  Shadow stack memory is always private and anonymous and so special
guard gap logic is not needed in a lot of caseis, but it can be mapped by
THP, so needs to be handled.

Link: https://lkml.kernel.org/r/20240326021656.202649-7-rick.p.edgecombe@intel.com
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Deepak Gupta <debug@rivosinc.com>
Cc: Guo Ren <guoren@kernel.org>
Cc: Helge Deller <deller@gmx.de>
Cc: H. Peter Anvin (Intel) <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Naveen N. Rao <naveen.n.rao@linux.ibm.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 7 insertions, 5 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index a37c8dd29d64..0ef1191e3be5 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1860,20 +1860,22 @@ __get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
 		 * so use shmem's get_unmapped_area in case it can be huge.
 		 */
 		get_area = shmem_get_unmapped_area;
-	} else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
-		/* Ensures that larger anonymous mappings are THP aligned. */
-		get_area = thp_get_unmapped_area;
 	}
 
 	/* Always treat pgoff as zero for anonymous memory. */
 	if (!file)
 		pgoff = 0;
 
-	if (get_area)
+	if (get_area) {
 		addr = get_area(file, addr, len, pgoff, flags);
-	else
+	} else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
+		/* Ensures that larger anonymous mappings are THP aligned. */
+		addr = thp_get_unmapped_area_vmflags(file, addr, len,
+						     pgoff, flags, vm_flags);
+	} else {
 		addr = mm_get_unmapped_area_vmflags(current->mm, file, addr, len,
 						    pgoff, flags, vm_flags);
+	}
 	if (IS_ERR_VALUE(addr))
 		return addr;