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    Subject[PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_SG


    On Thu, 19 Apr 2018, Michael S. Tsirkin wrote:

    > Maybe make it conditional on CONFIG_DEBUG_SG too?
    > Otherwise I think you just trigger a hard to debug memory corruption.

    OK, here I resend the patch with CONFIG_DEBUG_SG. With CONFIG_DEBUG_SG,
    the DMA API will print a stacktrace where the misuse happened, so it's
    much easier to debug than with CONFIG_DEBUG_VM.

    Fedora doesn't use CONFIG_DEBUG_SG in its default kernel (it only uses it
    in the debugging kernel), so users won't be hurt by this.



    From: Mikulas Patocka <mpatocka@redhat.com>
    Subject: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_SG

    The kvmalloc function tries to use kmalloc and falls back to vmalloc if
    kmalloc fails.

    Unfortunatelly, some kernel code has bugs - it uses kvmalloc and then
    uses DMA-API on the returned memory or frees it with kfree. Such bugs were
    found in the virtio-net driver, dm-integrity or RHEL7 powerpc-specific
    code.

    These bugs are hard to reproduce because vmalloc falls back to kmalloc
    only if memory is fragmented.

    In order to detect these bugs reliably I submit this patch that changes
    kvmalloc to always use vmalloc if CONFIG_DEBUG_SG is turned on.

    Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

    ---
    mm/util.c | 2 ++
    1 file changed, 2 insertions(+)

    Index: linux-2.6/mm/util.c
    ===================================================================
    --- linux-2.6.orig/mm/util.c 2018-04-18 15:46:23.000000000 +0200
    +++ linux-2.6/mm/util.c 2018-04-19 23:14:14.000000000 +0200
    @@ -395,6 +395,7 @@ EXPORT_SYMBOL(vm_mmap);
    */
    void *kvmalloc_node(size_t size, gfp_t flags, int node)
    {
    +#ifndef CONFIG_DEBUG_SG
    gfp_t kmalloc_flags = flags;
    void *ret;

    @@ -426,6 +427,7 @@ void *kvmalloc_node(size_t size, gfp_t f
    */
    if (ret || size <= PAGE_SIZE)
    return ret;
    +#endif

    return __vmalloc_node_flags_caller(size, node, flags,
    __builtin_return_address(0));
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