hugetlbfs: Ensure generic_hugetlb_get_unmapped_area() returns higher address than mmap_min_addr

generic_hugetlb_get_unmapped_area() was returning an address less than mmap_min_addr if the mmap argument addr, after alignment, was less than mmap_min_addr, causing mmap to fail.

This is because current generic_hugetlb_get_unmapped_area() code does not take into account mmap_min_addr.

This patch ensures that generic_hugetlb_get_unmapped_area() always returns an address that is greater than mmap_min_addr. Additionally, similar to generic_get_unmapped_area(), vm_end_gap() checks are included to maintain stack gap.

How to reproduce
================

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <unistd.h>

#define HUGEPAGE_SIZE (16 * 1024 * 1024)

int main() {

    void *addr = mmap((void *)-1, HUGEPAGE_SIZE,
                 PROT_READ | PROT_WRITE,
                 MAP_SHARED | MAP_ANONYMOUS | MAP_HUGETLB, -1, 0);
    if (addr == MAP_FAILED) {
        perror("mmap");
        exit(EXIT_FAILURE);
    }

    snprintf((char *)addr, HUGEPAGE_SIZE, "Hello, Huge Pages!");

    printf("%s\n", (char *)addr);

    if (munmap(addr, HUGEPAGE_SIZE) == -1) {
        perror("munmap");
        exit(EXIT_FAILURE);
    }

    return 0;

}

Result without fix
==================

 # cat /proc/meminfo |grep -i HugePages_Free
 HugePages_Free:       20
 # ./test
 mmap: Permission denied
 #

Result with fix
===============

 # cat /proc/meminfo |grep -i HugePages_Free
 HugePages_Free:       20
 # ./test
 Hello, Huge Pages!
 #

V3:
Changed subject prefix and commit message.

V2:
Added vm_end_gap() check.
https://lore.kernel.org/all/[email protected]/

V1:
https://lore.kernel.org/all/[email protected]/

Reported-by Pavithra Prakash <[email protected]>

---
 fs/hugetlbfs/inode.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

index 412f295acebe..cdd8e53ddd19 100644
@@ -222,13 +222,13 @@ generic_hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
 				  unsigned long flags)
 {
 	struct mm_struct *mm = current->mm;
-	struct vm_area_struct *vma;
+	struct vm_area_struct *vma, *prev;
 	struct hstate *h = hstate_file(file);
 	const unsigned long mmap_end = arch_get_mmap_end(addr, len, flags);
 
 	if (len & ~huge_page_mask(h))
 		return -EINVAL;
-	if (len > TASK_SIZE)
+	if (len > mmap_end - mmap_min_addr)
 		return -ENOMEM;
 
 	if (flags & MAP_FIXED) {

@@ -239,9 +239,10 @@ generic_hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
 
 	if (addr) {
 		addr = ALIGN(addr, huge_page_size(h));
-		vma = find_vma(mm, addr);
-		if (mmap_end - len >= addr &&
-		    (!vma || addr + len <= vm_start_gap(vma)))
+		vma = find_vma_prev(mm, addr, &prev);
+		if (mmap_end - len >= addr && addr >= mmap_min_addr &&
+		    (!vma || addr + len <= vm_start_gap(vma)) &&
+		    (!prev || addr >= vm_end_gap(prev)))
 			return addr;
 	}
 

2.43.5