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    Subject[PATCH v3] 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 kvmalloc falls back to vmalloc
    only if memory is fragmented.

    In order to detect these bugs reliably I submit this patch that changes
    kvmalloc to fall back to vmalloc with 1/2 probability if CONFIG_DEBUG_SG
    is turned on. CONFIG_DEBUG_SG is used, because it makes the DMA API layer
    verify the addresses passed to it, and so the user will get a reliable
    stacktrace.

    Some bugs (such as buffer overflows) are better detected
    with kmalloc code, so we must test the kmalloc path too.

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

    ---
    mm/util.c | 10 ++++++++++
    1 file changed, 10 insertions(+)

    Index: linux-2.6/mm/util.c
    ===================================================================
    --- linux-2.6.orig/mm/util.c 2018-04-23 00:12:05.000000000 +0200
    +++ linux-2.6/mm/util.c 2018-04-23 17:57:02.000000000 +0200
    @@ -14,6 +14,7 @@
    #include <linux/hugetlb.h>
    #include <linux/vmalloc.h>
    #include <linux/userfaultfd_k.h>
    +#include <linux/random.h>

    #include <asm/sections.h>
    #include <linux/uaccess.h>
    @@ -404,6 +405,12 @@ void *kvmalloc_node(size_t size, gfp_t f
    */
    WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);

    +#ifdef CONFIG_DEBUG_SG
    + /* Catch bugs when the caller uses DMA API on the result of kvmalloc. */
    + if (!(prandom_u32_max(2) & 1))
    + goto do_vmalloc;
    +#endif
    +
    /*
    * We want to attempt a large physically contiguous block first because
    * it is less likely to fragment multiple larger blocks and therefore
    @@ -427,6 +434,9 @@ void *kvmalloc_node(size_t size, gfp_t f
    if (ret || size <= PAGE_SIZE)
    return ret;

    +#ifdef CONFIG_DEBUG_SG
    +do_vmalloc:
    +#endif
    return __vmalloc_node_flags_caller(size, node, flags,
    __builtin_return_address(0));
    }
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