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

Ard Biesheuvel
2020-03-03T10:21:14Z

On Tue, 3 Mar 2020 at 11:14, Vladis Dronov <vdronov@redhat.com> wrote:
>
> 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);

I'd assume that the memcpy() is not executed if var_data_size > 1024,
and var->DataSize is not assigned in that case either.

> 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
>