[PATCH] efivarfs: Rate limit statfs() handler

Ravi reports that statfs() may be called by unprivileged users on the
efivarfs mount point, which may result in a flood of calls to the
QueryVariableInfo() runtime service. These calls are disproportionately
costly on x86 systems where the variable store is backed by SMM, as each
SMM entry requires a rendez-vous of all the CPUs.

So rate limit the calls to QueryVariableInfo() at twice per second, and
return the most recently obtained values for calls that are elided.

Cc: <[email protected]>
Cc: Anisse Astier <[email protected]>
Reported-by: Ravi Bangoria <[email protected]>
Fixes: d86ff3333cb1 ("efivarfs: expose used and total size")
Signed-off-by: Ard Biesheuvel <[email protected]>
---
This supersedes [0], which had some issues, the main one being that a
cache that requires explicit invalidation may go out of sync due to
direct calls to SetVariable() made by other drivers.

[0] https://lore.kernel.org/all/[email protected]/

 fs/efivarfs/super.c | 30 ++++++++++++++++----
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c
index 733c19571f1c..8d33f11db2a1 100644
--- a/fs/efivarfs/super.c
+++ b/fs/efivarfs/super.c
@@ -89,12 +89,30 @@ static int efivarfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 	/* Some UEFI firmware does not implement QueryVariableInfo() */
 	storage_space = remaining_space = 0;
 	if (efi_rt_services_supported(EFI_RT_SUPPORTED_QUERY_VARIABLE_INFO)) {
-		status = efivar_query_variable_info(attr, &storage_space,
-						    &remaining_space,
-						    &max_variable_size);
-		if (status != EFI_SUCCESS && status != EFI_UNSUPPORTED)
-			pr_warn_ratelimited("query_variable_info() failed: 0x%lx\n",
-					    status);
+		static DEFINE_RATELIMIT_STATE(_rs, 2 * HZ, 5);
+		static u64 storage, remaining;
+		static DEFINE_SPINLOCK(lock);
+
+		if (!__ratelimit(&_rs)) {
+			ratelimit_set_flags(&_rs, RATELIMIT_MSG_ON_RELEASE);
+
+			spin_lock(&lock);
+			storage_space = storage;
+			remaining_space = remaining;
+			spin_unlock(&lock);
+		} else {
+			status = efivar_query_variable_info(attr, &storage_space,
+							    &remaining_space,
+							    &max_variable_size);
+			if (status != EFI_SUCCESS && status != EFI_UNSUPPORTED)
+				pr_warn("query_variable_info() failed: 0x%lx\n",
+					status);
+
+			spin_lock(&lock);
+			storage = storage_space;
+			remaining = remaining_space;
+			spin_unlock(&lock);
+		}
 	}
 
 	/*
-- 
2.55.0.699.gb54405d56f-goog