From: Guanghui Feng <[email protected]> need_sync is a per-IOMMU flag shared by all domains and devices behind that IOMMU. It is set whenever a command is queued with sync == true and cleared when a completion-wait (CWAIT) command is queued. However, a cleared need_sync only means that a covering CWAIT has been queued, not that all previously queued commands have actually completed in hardware.
iommu_completion_wait() read need_sync locklessly and returned early when it was false. This breaks the "block until all previously queued commands have completed" contract in a multi-CPU scenario: CPU2: queue inv-B => need_sync = true CPU1: queue CWAIT(N); need_sync = false; then wait_on_sem(N) CPU2: read need_sync == false => return 0 (no wait!) CPU2 returns without waiting for any sequence number even though its inv-B may not have completed yet (CWAIT(N), queued after inv-B, has not been signaled). CPU2 then proceeds to, for example, free page-table pages while the IOMMU can still walk stale translations, opening a use-after-free window. This is a logical race in the meaning of the flag, not a memory-visibility issue, so barriers alone do not help. Fix it without losing the optimization of avoiding redundant CWAIT commands: take iommu->lock before testing need_sync, and when it is false do not return early but wait for the last allocated sequence number (cmd_sem_val). Since need_sync == false implies no sync command was queued after the last CWAIT, that CWAIT is FIFO-ordered after every not-yet-completed command, so waiting for its sequence number guarantees all prior commands (possibly queued by another CPU) have completed. The common path with pending work is unchanged and no extra hardware command is issued. Signed-off-by: Guanghui Feng <[email protected]> Fixes: 815b33fdc279 ("x86/amd-iommu: Cleanup completion-wait handling") Reviewed-by: Vasant Hegde <[email protected]> Signed-off-by: Will Deacon <[email protected]> (cherry picked from commit 1e75a8255f11c81fb07e81e5029cfd75804350a0) https://virtuozzo.atlassian.net/browse/VSTOR-154424 Feature: fix ms/iommu Signed-off-by: Denis V. Lunev <[email protected]> --- drivers/iommu/amd/iommu.c | 22 ++++++++++++++++------ 1 file changed, 16 insertions(+), 6 deletions(-) diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c index 373596fa07c0..91cf277f1293 100644 --- a/drivers/iommu/amd/iommu.c +++ b/drivers/iommu/amd/iommu.c @@ -1408,11 +1408,23 @@ static int iommu_completion_wait(struct amd_iommu *iommu) int ret; u64 data; - if (!iommu->need_sync) - return 0; - raw_spin_lock_irqsave(&iommu->lock, flags); + if (!iommu->need_sync) { + /* + * No command has been queued since the last completion-wait. + * A concurrent CPU may have already queued that CWAIT and + * cleared need_sync; need_sync == false only means a covering + * CWAIT is queued, not that all prior commands have completed. + * Wait for the last allocated sequence number so that any + * command queued before this call (possibly on another CPU) + * is guaranteed to have completed before returning. + */ + data = iommu->cmd_sem_val; + raw_spin_unlock_irqrestore(&iommu->lock, flags); + return wait_on_sem(iommu, data); + } + data = get_cmdsem_val(iommu); build_completion_wait(&cmd, iommu, data); @@ -1422,9 +1434,7 @@ static int iommu_completion_wait(struct amd_iommu *iommu) if (ret) return ret; - ret = wait_on_sem(iommu, data); - - return ret; + return wait_on_sem(iommu, data); } static void domain_flush_complete(struct protection_domain *domain) -- 2.53.0 _______________________________________________ Devel mailing list [email protected] https://lists.openvz.org/mailman/listinfo/devel
