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Subject[PATCH] Bug in kernel/fork.c in 2.4.4 kernel
In working on thread core dumps I've stumbled across a minor bug in the
generic `fork' code. The problem code is in routine `copy_mm' in
`kernel/fork.c':

/* Copy the current MM stuff.. */
memcpy(mm, oldmm, sizeof(*mm));
.
.
.
if (retval)
goto free_pt;

/*
* child gets a private LDT (if there was an LDT in the parent)
*/
copy_segments(tsk, mm);
.
.
.
free_pt:
mmput(mm);

The new `mm' doesn't get it's own private version of it's `context'
field until after the call to `copy_segments'. At the `goto' the pointer
`mm' points to a copy of the old `mm_struct', including a copy of the
`context' field. `mmput' will call `release_segments' which will free
the memory pointed to by the `context' field. Later, when `oldmm' is
freed, the kernel will try to free the same memory twice - very bad.

Admittedly, this will only occur on an obscure error condition but
it should be fixed. The attached patch fixes the problem by zeroing
out the `context' field immediately after the `mm' structure is
copied.
--
Don Dugger
"Censeo Toto nos in Kansa esse decisse." - D. Gale
n0ano@valinux.com
Ph: 303/938-9838


--- linux-2.4.4-ref/kernel/fork.c Thu Apr 26 07:11:17 2001
+++ linux/kernel/fork.c Wed May 2 14:39:38 2001
@@ -311,6 +311,10 @@

/* Copy the current MM stuff.. */
memcpy(mm, oldmm, sizeof(*mm));
+
+ /* Clear new context for now */
+ memset(&mm->context, 0, sizeof(mm->context));
+
if (!mm_init(mm))
goto fail_nomem;

-
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