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

Vladis Dronov
2020-03-03T10:14:19Z

Hello, Ard, all,

----- Original Message -----
> From: "Ard Biesheuvel" <ardb@kernel.org>
> To: "Vladis Dronov" <vdronov@redhat.com>
> Cc: "linux-efi" <linux-efi@vger.kernel.org>, "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
> Sent: Tuesday, March 3, 2020 10:15:41 AM
> Subject: Re: [PATCH] efi: fix a race and a buffer overflow while reading efivars via sysfs
>
> On Tue, 3 Mar 2020 at 09:55, Vladis Dronov <vdronov@redhat.com> wrote:
> >
> > 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>
>
> Wouldn't it be easier to pass a var_data_size stack variable into
> efivar_entry_get(), and only update the value in 'var' if it is <=
> 1024?
>

I was thinking about this approach, but this way we still do not protect
var from a concurrent access. For example, efivar_data_read() can race
with itself:

// reading var size 5
efivar_data_read()
efivar_entry_get()
// reading the same var
efivar_data_read()
var->DataSize = 1024;
efivar_entry_get()
// var->DataSize is SUDDENLY 1024
memcpy(buf, var->Data, var->DataSize);
return var->DataSize;

Also efivar read functions still can race with the write function
efivar_store_raw(). Surely, the race window is much smaller but it is there.
I strongly believe we need to protect all data accesses here with a lock.

May be not in a way I suggest, may be by a per-var mutex, but I believe
this is overcomplication.

> > ---
> > 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