[PATCH kernel v2 3/6] KVM: PPC: Account TCE-containing pages in locked_vm

[Alexey Kardashevskiy]

At the moment pages used for TCE tables (in addition to pages addressed
by TCEs) are not counted in locked_vm counter so a malicious userspace
tool can call ioctl(KVM_CREATE_SPAPR_TCE) as many times as RLIMIT_NOFILE and
lock a lot of memory.

This adds counting for pages used for TCE tables.

This counts the number of pages required for a table plus pages for
the kvmppc_spapr_tce_table struct (TCE table descriptor) itself.

This changes release_spapr_tce_table() to store @npages on stack to
avoid calling kvmppc_stt_npages() in the loop (tiny optimization,
probably).

This does not change the amount of (de)allocated memory.

Signed-off-by: Alexey Kardashevskiy <a...@ozlabs.ru>
---
Changes:
v2:
* switched from long to unsigned long types
* added WARN_ON_ONCE() in locked_vm decrement case
---
 arch/powerpc/kvm/book3s_64_vio.c | 55 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 52 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
index 9526c34..ea498b4 100644
--- a/arch/powerpc/kvm/book3s_64_vio.c
+++ b/arch/powerpc/kvm/book3s_64_vio.c
@@ -39,19 +39,62 @@
 
 #define TCES_PER_PAGE  (PAGE_SIZE / sizeof(u64))
 
-static long kvmppc_stt_npages(unsigned long window_size)
+static unsigned long kvmppc_stt_npages(unsigned long window_size)
 {
        return ALIGN((window_size >> SPAPR_TCE_SHIFT)
                     * sizeof(u64), PAGE_SIZE) / PAGE_SIZE;
 }
 
+static long kvmppc_account_memlimit(unsigned long npages, bool inc)
+{
+       long ret = 0;
+       const unsigned long bytes = sizeof(struct kvmppc_spapr_tce_table) +
+                       (npages * sizeof(struct page *));
+       const unsigned long stt_pages = ALIGN(bytes, PAGE_SIZE) / PAGE_SIZE;
+
+       if (!current || !current->mm)
+               return ret; /* process exited */
+
+       npages += stt_pages;
+
+       down_write(&current->mm->mmap_sem);
+
+       if (inc) {
+               unsigned long locked, lock_limit;
+
+               locked = current->mm->locked_vm + npages;
+               lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
+               if (locked > lock_limit && !capable(CAP_IPC_LOCK))
+                       ret = -ENOMEM;
+               else
+                       current->mm->locked_vm += npages;
+       } else {
+               if (WARN_ON_ONCE(npages > current->mm->locked_vm))
+                       npages = current->mm->locked_vm;
+
+               current->mm->locked_vm -= npages;
+       }
+
+       pr_debug("[%d] RLIMIT_MEMLOCK KVM %c%ld %ld/%ld%s\n", current->pid,
+                       inc ? '+' : '-',
+                       npages << PAGE_SHIFT,
+                       current->mm->locked_vm << PAGE_SHIFT,
+                       rlimit(RLIMIT_MEMLOCK),
+                       ret ? " - exceeded" : "");
+
+       up_write(&current->mm->mmap_sem);
+
+       return ret;
+}
+
 static void release_spapr_tce_table(struct rcu_head *head)
 {
        struct kvmppc_spapr_tce_table *stt = container_of(head,
                        struct kvmppc_spapr_tce_table, rcu);
        int i;
+       unsigned long npages = kvmppc_stt_npages(stt->window_size);
 
-       for (i = 0; i < kvmppc_stt_npages(stt->window_size); i++)
+       for (i = 0; i < npages; i++)
                __free_page(stt->pages[i]);
 
        kfree(stt);
@@ -89,6 +132,7 @@ static int kvm_spapr_tce_release(struct inode *inode, struct 
file *filp)
 
        kvm_put_kvm(stt->kvm);
 
+       kvmppc_account_memlimit(kvmppc_stt_npages(stt->window_size), false);
        call_rcu(&stt->rcu, release_spapr_tce_table);
 
        return 0;
@@ -103,7 +147,7 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
                                   struct kvm_create_spapr_tce *args)
 {
        struct kvmppc_spapr_tce_table *stt = NULL;
-       long npages;
+       unsigned long npages;
        int ret = -ENOMEM;
        int i;
 
@@ -114,6 +158,11 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
        }
 
        npages = kvmppc_stt_npages(args->window_size);
+       ret = kvmppc_account_memlimit(npages, true);
+       if (ret) {
+               stt = NULL;
+               goto fail;
+       }
 
        stt = kzalloc(sizeof(*stt) + npages * sizeof(struct page *),
                      GFP_KERNEL);
-- 
2.5.0.rc3

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