[PATCH] efi: fix a race and a buffer overflow while reading efivars via sysfs

Vladis Dronov
2020-03-03T08:55:56Z

There is a race and a buffer overflow corrupting a kernel memory while
reading an efi variable with a size more than 1024 bytes via the older
sysfs method. This happens because accessing struct efi_variable in
efivar_{attr,size,data}_read() and friends is not protected from
a concurrent access leading to a kernel memory corruption and, at best,
to a crash. The race scenario is the following:

CPU0: CPU1:
efivar_attr_read()
var->DataSize = 1024;
efivar_entry_get(... &var->DataSize)
down_interruptible(&efivars_lock)
efivar_attr_read() // same efi var
var->DataSize = 1024;
efivar_entry_get(... &var->DataSize)
down_interruptible(&efivars_lock)
virt_efi_get_variable()
// returns EFI_BUFFER_TOO_SMALL but
// var->DataSize is set to a real
// var size more than 1024 bytes
up(&efivars_lock)
virt_efi_get_variable()
// called with var->DataSize set
// to a real var size, returns
// successfully and overwrites
// a 1024-bytes kernel buffer
up(&efivars_lock)

This can be reproduced by concurrent reading of an efi variable which size
is more than 1024 bytes:

ts# for cpu in $(seq 0 $(nproc --ignore=1)); do ( taskset -c $cpu \
cat /sys/firmware/efi/vars/KEKDefault*/size & ) ; done

Fix this by protecting struct efi_variable access by efivars_lock by using
efivar_entry_iter_begin()/efivar_entry_iter_end(). Brush up and unify
efivar_entry_[gs]et() and __efivar_entry_[gs]et() for this. This looks
simpler than introducing a separate lock for every struct efi_variable.

Also fix the same race in efivar_store_raw() and efivar_show_raw(). The
call in efi_pstore_read_func() is protected like this already.

Reported-by: Bob Sanders <bob.sanders@hpe.com> and the LTP testsuite
Signed-off-by: Vladis Dronov <vdronov@redhat.com>
---
drivers/firmware/efi/efi-pstore.c | 2 +-
drivers/firmware/efi/efivars.c | 72 ++++++++++++++++++++++++-------
drivers/firmware/efi/vars.c | 47 ++++++++++++--------
include/linux/efi.h | 2 +
4 files changed, 90 insertions(+), 33 deletions(-)

diff --git a/drivers/firmware/efi/efi-pstore.c b/drivers/firmware/efi/efi-pstore.c
index 9ea13e8d12ec..e4767a7ce973 100644
--- a/drivers/firmware/efi/efi-pstore.c
+++ b/drivers/firmware/efi/efi-pstore.c
@@ -161,7 +161,7 @@ static int efi_pstore_scan_sysfs_exit(struct efivar_entry *pos,
*
* @record: pstore record to pass to callback
*
- * You MUST call efivar_enter_iter_begin() before this function, and
+ * You MUST call efivar_entry_iter_begin() before this function, and
* efivar_entry_iter_end() afterwards.
*
*/
diff --git a/drivers/firmware/efi/efivars.c b/drivers/firmware/efi/efivars.c
index 7576450c8254..f415cf863ee0 100644
--- a/drivers/firmware/efi/efivars.c
+++ b/drivers/firmware/efi/efivars.c
@@ -88,9 +88,15 @@ efivar_attr_read(struct efivar_entry *entry, char *buf)
if (!entry || !buf)
return -EINVAL;

+ if (efivar_entry_iter_begin())
+ return -EINTR;
+
var->DataSize = 1024;
- if (efivar_entry_get(entry, &var->Attributes, &var->DataSize, var->Data))
+ if (__efivar_entry_get(entry, &var->Attributes, &var->DataSize,
+ var->Data)) {
+ efivar_entry_iter_end();
return -EIO;
+ }

if (var->Attributes & EFI_VARIABLE_NON_VOLATILE)
str += sprintf(str, "EFI_VARIABLE_NON_VOLATILE\n");
@@ -109,6 +115,8 @@ efivar_attr_read(struct efivar_entry *entry, char *buf)
"EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS\n");
if (var->Attributes & EFI_VARIABLE_APPEND_WRITE)
str += sprintf(str, "EFI_VARIABLE_APPEND_WRITE\n");
+
+ efivar_entry_iter_end();
return str - buf;
}

@@ -121,11 +129,19 @@ efivar_size_read(struct efivar_entry *entry, char *buf)
if (!entry || !buf)
return -EINVAL;

+ if (efivar_entry_iter_begin())
+ return -EINTR;
+
var->DataSize = 1024;
- if (efivar_entry_get(entry, &var->Attributes, &var->DataSize, var->Data))
+ if (__efivar_entry_get(entry, &var->Attributes, &var->DataSize,
+ var->Data)) {
+ efivar_entry_iter_end();
return -EIO;
+ }

str += sprintf(str, "0x%lx\n", var->DataSize);
+
+ efivar_entry_iter_end();
return str - buf;
}

@@ -137,11 +153,19 @@ efivar_data_read(struct efivar_entry *entry, char *buf)
if (!entry || !buf)
return -EINVAL;

+ if (efivar_entry_iter_begin())
+ return -EINTR;
+
var->DataSize = 1024;
- if (efivar_entry_get(entry, &var->Attributes, &var->DataSize, var->Data))
+ if (__efivar_entry_get(entry, &var->Attributes, &var->DataSize,
+ var->Data)) {
+ efivar_entry_iter_end();
return -EIO;
+ }

memcpy(buf, var->Data, var->DataSize);
+
+ efivar_entry_iter_end();
return var->DataSize;
}

@@ -197,13 +221,21 @@ efivar_store_raw(struct efivar_entry *entry, const char *buf, size_t count)
efi_guid_t vendor;
u32 attributes;
u8 *data;
- int err;
+ int err = 0;
+
+ if (!entry || !buf)
+ return -EINVAL;
+
+ if (efivar_entry_iter_begin())
+ return -EINTR;

if (in_compat_syscall()) {
struct compat_efi_variable *compat;

- if (count != sizeof(*compat))
- return -EINVAL;
+ if (count != sizeof(*compat)) {
+ err = -EINVAL;
+ goto out;
+ }

compat = (struct compat_efi_variable *)buf;
attributes = compat->Attributes;
@@ -214,12 +246,14 @@ efivar_store_raw(struct efivar_entry *entry, const char *buf, size_t count)

err = sanity_check(var, name, vendor, size, attributes, data);
if (err)
- return err;
+ goto out;

copy_out_compat(&entry->var, compat);
} else {
- if (count != sizeof(struct efi_variable))
- return -EINVAL;
+ if (count != sizeof(struct efi_variable)) {
+ err = -EINVAL;
+ goto out;
+ }

new_var = (struct efi_variable *)buf;

@@ -231,18 +265,20 @@ efivar_store_raw(struct efivar_entry *entry, const char *buf, size_t count)

err = sanity_check(var, name, vendor, size, attributes, data);
if (err)
- return err;
+ goto out;

memcpy(&entry->var, new_var, count);
}

- err = efivar_entry_set(entry, attributes, size, data, NULL);
+ err = __efivar_entry_set(entry, attributes, size, data, NULL);
if (err) {
printk(KERN_WARNING "efivars: set_variable() failed: status=%d\n", err);
- return -EIO;
+ err = -EIO;
}

- return count;
+out:
+ efivar_entry_iter_end();
+ return err ?: count;
}

static ssize_t
@@ -255,10 +291,15 @@ efivar_show_raw(struct efivar_entry *entry, char *buf)
if (!entry || !buf)
return 0;

+ if (efivar_entry_iter_begin())
+ return -EINTR;
+
var->DataSize = 1024;
- if (efivar_entry_get(entry, &entry->var.Attributes,
- &entry->var.DataSize, entry->var.Data))
+ if (__efivar_entry_get(entry, &var->Attributes, &var->DataSize,
+ var->Data)) {
+ efivar_entry_iter_end();
return -EIO;
+ }

if (in_compat_syscall()) {
compat = (struct compat_efi_variable *)buf;
@@ -276,6 +317,7 @@ efivar_show_raw(struct efivar_entry *entry, char *buf)
memcpy(buf, var, size);
}

+ efivar_entry_iter_end();
return size;
}

diff --git a/drivers/firmware/efi/vars.c b/drivers/firmware/efi/vars.c
index 436d1776bc7b..4a47e20a7667 100644
--- a/drivers/firmware/efi/vars.c
+++ b/drivers/firmware/efi/vars.c
@@ -636,7 +636,7 @@ int efivar_entry_delete(struct efivar_entry *entry)
EXPORT_SYMBOL_GPL(efivar_entry_delete);

/**
- * efivar_entry_set - call set_variable()
+ * __efivar_entry_set - call set_variable()
* @entry: entry containing the EFI variable to write
* @attributes: variable attributes
* @size: size of @data buffer
@@ -655,8 +655,12 @@ EXPORT_SYMBOL_GPL(efivar_entry_delete);
* Returns 0 on success, -EINTR if we can't grab the semaphore,
* -EEXIST if a lookup is performed and the entry already exists on
* the list, or a converted EFI status code if set_variable() fails.
+ *
+ * The caller MUST call efivar_entry_iter_begin() and
+ * efivar_entry_iter_end() before and after the invocation of this
+ * function, respectively.
*/
-int efivar_entry_set(struct efivar_entry *entry, u32 attributes,
+int __efivar_entry_set(struct efivar_entry *entry, u32 attributes,
unsigned long size, void *data, struct list_head *head)
{
const struct efivar_operations *ops;
@@ -664,9 +668,6 @@ int efivar_entry_set(struct efivar_entry *entry, u32 attributes,
efi_char16_t *name = entry->var.VariableName;
efi_guid_t vendor = entry->var.VendorGuid;

- if (down_interruptible(&efivars_lock))
- return -EINTR;
-
if (!__efivars) {
up(&efivars_lock);
return -EINVAL;
@@ -682,10 +683,28 @@ int efivar_entry_set(struct efivar_entry *entry, u32 attributes,
status = ops->set_variable(name, &vendor,
attributes, size, data);

- up(&efivars_lock);
-
return efi_status_to_err(status);
+}
+EXPORT_SYMBOL_GPL(__efivar_entry_set);

+/**
+ * efivar_entry_set - call set_variable()
+ *
+ * This function takes efivars_lock and calls __efivar_entry_set()
+ */
+int efivar_entry_set(struct efivar_entry *entry, u32 attributes,
+ unsigned long size, void *data, struct list_head *head)
+{
+ int ret;
+
+ if (down_interruptible(&efivars_lock))
+ return -EINTR;
+
+ ret = __efivar_entry_set(entry, attributes, size, data, head);
+
+ up(&efivars_lock);
+
+ return ret;
}
EXPORT_SYMBOL_GPL(efivar_entry_set);

@@ -914,22 +933,16 @@ EXPORT_SYMBOL_GPL(__efivar_entry_get);
int efivar_entry_get(struct efivar_entry *entry, u32 *attributes,
unsigned long *size, void *data)
{
- efi_status_t status;
+ int ret;

if (down_interruptible(&efivars_lock))
return -EINTR;

- if (!__efivars) {
- up(&efivars_lock);
- return -EINVAL;
- }
+ ret = __efivar_entry_get(entry, attributes, size, data);

- status = __efivars->ops->get_variable(entry->var.VariableName,
- &entry->var.VendorGuid,
- attributes, size, data);
up(&efivars_lock);

- return efi_status_to_err(status);
+ return ret;
}
EXPORT_SYMBOL_GPL(efivar_entry_get);

@@ -1071,7 +1084,7 @@ EXPORT_SYMBOL_GPL(efivar_entry_iter_end);
* entry on the list. It is safe for @func to remove entries in the
* list via efivar_entry_delete().
*
- * You MUST call efivar_enter_iter_begin() before this function, and
+ * You MUST call efivar_entry_iter_begin() before this function, and
* efivar_entry_iter_end() afterwards.
*
* It is possible to begin iteration from an arbitrary entry within
diff --git a/include/linux/efi.h b/include/linux/efi.h
index 7efd7072cca5..5c3db088d375 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -1414,6 +1414,8 @@ int __efivar_entry_get(struct efivar_entry *entry, u32 *attributes,
unsigned long *size, void *data);
int efivar_entry_get(struct efivar_entry *entry, u32 *attributes,
unsigned long *size, void *data);
+int __efivar_entry_set(struct efivar_entry *entry, u32 attributes,
+ unsigned long size, void *data, struct list_head *head);
int efivar_entry_set(struct efivar_entry *entry, u32 attributes,
unsigned long size, void *data, struct list_head *head);
int efivar_entry_set_get_size(struct efivar_entry *entry, u32 attributes,
--
2.20.1