Fix incorrect user space access locking in mincore()

Doug Chapman noticed that mincore() will doa "copy_to_user()" of the
result while holding the mmap semaphore for reading, which is a big
no-no.  While a recursive read-lock on a semaphore in the case of a page
fault happens to work, we don't actually allow them due to deadlock
schenarios with writers due to fairness issues.

Doug and Marcel sent in a patch to fix it, but I decided to just rewrite
the mess instead - not just fixing the locking problem, but making the
code smaller and (imho) much easier to understand.

Cc: Doug Chapman <dchapman@redhat.com>
Cc: Marcel Holtmann <holtmann@redhat.com>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 86 insertions, 104 deletions

diff --git a/mm/mincore.c b/mm/mincore.c
index 72890780c1c9..b44d7f875cb6 100644
--- a/mm/mincore.c
+++ b/mm/mincore.c
@@ -1,7 +1,7 @@
 /*
  *      linux/mm/mincore.c
  *
- * Copyright (C) 1994-1999  Linus Torvalds
+ * Copyright (C) 1994-2006  Linus Torvalds
  */
 
 /*
@@ -38,46 +38,60 @@ static unsigned char mincore_page(struct vm_area_struct * vma,
         return present;
 }
 
-static long mincore_vma(struct vm_area_struct * vma,
-        unsigned long start, unsigned long end, unsigned char __user * vec)
+/*
+ * Do a chunk of "sys_mincore()". We've already checked
+ * all the arguments, we hold the mmap semaphore: we should
+ * just return the amount of info we're asked for.
+ */
+static long do_mincore(unsigned long addr, unsigned char *vec, unsigned long pages)
 {
-        long error, i, remaining;
-        unsigned char * tmp;
+        unsigned long i, nr, pgoff;
+        struct vm_area_struct *vma = find_vma(current->mm, addr);
 
-        error = -ENOMEM;
-        if (!vma->vm_file)
-                return error;
-
-        start = ((start - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
-        if (end > vma->vm_end)
-                end = vma->vm_end;
-        end = ((end - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
+        /*
+         * find_vma() didn't find anything: the address
+         * is above everything we have mapped.
+         */
+        if (!vma) {
+                memset(vec, 0, pages);
+                return pages;
+        }
 
-        error = -EAGAIN;
-        tmp = (unsigned char *) __get_free_page(GFP_KERNEL);
-        if (!tmp)
-                return error;
+        /*
+         * find_vma() found something, but we might be
+         * below it: check for that.
+         */
+        if (addr < vma->vm_start) {
+                unsigned long gap = (vma->vm_start - addr) >> PAGE_SHIFT;
+                if (gap > pages)
+                        gap = pages;
+                memset(vec, 0, gap);
+                return gap;
+        }
 
-        /* (end - start) is # of pages, and also # of bytes in "vec */
-        remaining = (end - start),
+        /*
+         * Ok, got it. But check whether it's a segment we support
+         * mincore() on. Right now, we don't do any anonymous mappings.
+         */
+        if (!vma->vm_file)
+                return -ENOMEM;
 
-        error = 0;
-        for (i = 0; remaining > 0; remaining -= PAGE_SIZE, i++) {
-                int j = 0;
-                long thispiece = (remaining < PAGE_SIZE) ?
-                                                remaining : PAGE_SIZE;
+        /*
+         * Calculate how many pages there are left in the vma, and
+         * what the pgoff is for our address.
+         */
+        nr = (vma->vm_end - addr) >> PAGE_SHIFT;
+        if (nr > pages)
+                nr = pages;
 
-                while (j < thispiece)
-                        tmp[j++] = mincore_page(vma, start++);
+        pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
+        pgoff += vma->vm_pgoff;
 
-                if (copy_to_user(vec + PAGE_SIZE * i, tmp, thispiece)) {
-                        error = -EFAULT;
-                        break;
-                }
-        }
+        /* And then we just fill the sucker in.. */
+        for (i = 0 ; i < nr; i++, pgoff++)
+                vec[i] = mincore_page(vma, pgoff);
 
-        free_page((unsigned long) tmp);
-        return error;
+        return nr;
 }
 
 /*
@@ -107,82 +121,50 @@ static long mincore_vma(struct vm_area_struct * vma,
 asmlinkage long sys_mincore(unsigned long start, size_t len,
         unsigned char __user * vec)
 {
-        int index = 0;
-        unsigned long end, limit;
-        struct vm_area_struct * vma;
-        size_t max;
-        int unmapped_error = 0;
-        long error;
-
-        /* check the arguments */
-        if (start & ~PAGE_CACHE_MASK)
-                goto einval;
-
-        limit = TASK_SIZE;
-        if (start >= limit)
-                goto enomem;
-
-        if (!len)
-                return 0;
+        long retval;
+        unsigned long pages;
+        unsigned char *tmp;
 
-        max = limit - start;
-        len = PAGE_CACHE_ALIGN(len);
-        if (len > max || !len)
-                goto enomem;
-
-        end = start + len;
+        /* Check the start address: needs to be page-aligned.. */
+        if (start & ~PAGE_CACHE_MASK)
+                return -EINVAL;
 
-        /* check the output buffer whilst holding the lock */
-        error = -EFAULT;
-        down_read(&current->mm->mmap_sem);
+        /* ..and we need to be passed a valid user-space range */
+        if (!access_ok(VERIFY_READ, (void __user *) start, len))
+                return -ENOMEM;
 
-        if (!access_ok(VERIFY_WRITE, vec, len >> PAGE_SHIFT))
-                goto out;
+        /* This also avoids any overflows on PAGE_CACHE_ALIGN */
+        pages = len >> PAGE_SHIFT;
+        pages += (len & ~PAGE_MASK) != 0;
 
-        /*
-         * If the interval [start,end) covers some unmapped address
-         * ranges, just ignore them, but return -ENOMEM at the end.
-         */
-        error = 0;
-
-        vma = find_vma(current->mm, start);
-        while (vma) {
-                /* Here start < vma->vm_end. */
-                if (start < vma->vm_start) {
-                        unmapped_error = -ENOMEM;
-                        start = vma->vm_start;
-                }
+        if (!access_ok(VERIFY_WRITE, vec, pages))
+                return -EFAULT;
 
-                /* Here vma->vm_start <= start < vma->vm_end. */
-                if (end <= vma->vm_end) {
-                        if (start < end) {
-                                error = mincore_vma(vma, start, end,
-                                                        &vec[index]);
-                                if (error)
-                                        goto out;
-                        }
-                        error = unmapped_error;
-                        goto out;
+        tmp = (void *) __get_free_page(GFP_USER);
+        if (!tmp)
+                return -ENOMEM;
+
+        retval = 0;
+        while (pages) {
+                /*
+                 * Do at most PAGE_SIZE entries per iteration, due to
+                 * the temporary buffer size.
+                 */
+                down_read(&current->mm->mmap_sem);
+                retval = do_mincore(start, tmp, max(pages, PAGE_SIZE));
+                up_read(&current->mm->mmap_sem);
+
+                if (retval <= 0)
+                        break;
+                if (copy_to_user(vec, tmp, retval)) {
+                        retval = -EFAULT;
+                        break;
                 }
-
-                /* Here vma->vm_start <= start < vma->vm_end < end. */
-                error = mincore_vma(vma, start, vma->vm_end, &vec[index]);
-                if (error)
-                        goto out;
-                index += (vma->vm_end - start) >> PAGE_CACHE_SHIFT;
-                start = vma->vm_end;
-                vma = vma->vm_next;
+                pages -= retval;
+                vec += retval;
+                start += retval << PAGE_SHIFT;
+                retval = 0;
         }
-
-        /* we found a hole in the area queried if we arrive here */
-        error = -ENOMEM;
-
-out:
-        up_read(&current->mm->mmap_sem);
-        return error;
-
-einval:
-        return -EINVAL;
-enomem:
-        return -ENOMEM;
+        free_page((unsigned long) tmp);
+        return retval;
 }