By default, the efi-pstore backend hardcode the UEFI variable size
as 1024 bytes. The historical reasons for that were discussed by
Ard in threads [0][1]:
"there is some cargo cult from prehistoric EFI times going
on here, it seems. Or maybe just misinterpretation of the maximum
size for the variable *name* vs the variable itself.".
"OVMF has
OvmfPkg/OvmfPkgX64.dsc:
gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x2000
OvmfPkg/OvmfPkgX64.dsc:
gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x8400
where the first one is without secure boot and the second with secure
boot. Interestingly, the default is
gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x400
so this is probably where this 1k number comes from."
With that, and since there is not such a limit in the UEFI spec, we
have the confidence to hereby add a module parameter to enable advanced
users to change the UEFI record size for efi-pstore data collection,
this way allowing a much easier reading of the collected log, which is
not scattered anymore among many small files.
Through empirical analysis we observed that extreme low values (like 8
bytes) could eventually cause writing issues, so given that and the OVMF
default discussed, we limited the minimum value to 1024 bytes, which also
is still the default.
[0] https://lore.kernel.org/lkml/CAMj1kXF4UyRMh2Y_KakeNBHvkHhTtavASTAxXinDO1rhPe_wYg@mail.gmail.com/
[1] https://lore.kernel.org/lkml/CAMj1kXFy-2KddGu+dgebAdU9v2sindxVoiHLWuVhqYw+R=kqng@mail.gmail.com/
Cc: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
---
V2:
- Fixed a memory corruption bug in the code (that wasn't causing
trouble before due to the fixed sized of record_size), thanks
Ard for spotting this!
- Added Ard's archeology in the commit message plus a comment
with the reasoning behind the minimum value.
drivers/firmware/efi/efi-pstore.c | 23 +++++++++++++++++------
1 file changed, 17 insertions(+), 6 deletions(-)
diff --git a/drivers/firmware/efi/efi-pstore.c b/drivers/firmware/efi/efi-pstore.c
index 97a9e84840a0..827e32427ddb 100644
--- a/drivers/firmware/efi/efi-pstore.c
+++ b/drivers/firmware/efi/efi-pstore.c
@@ -10,7 +10,9 @@ MODULE_IMPORT_NS(EFIVAR);
#define DUMP_NAME_LEN 66
-#define EFIVARS_DATA_SIZE_MAX 1024
+static unsigned int record_size = 1024;
+module_param(record_size, uint, 0444);
+MODULE_PARM_DESC(record_size, "size of each pstore UEFI var (in bytes, min/default=1024)");
static bool efivars_pstore_disable =
IS_ENABLED(CONFIG_EFI_VARS_PSTORE_DEFAULT_DISABLE);
@@ -30,7 +32,7 @@ static int efi_pstore_open(struct pstore_info *psi)
if (err)
return err;
- psi->data = kzalloc(EFIVARS_DATA_SIZE_MAX, GFP_KERNEL);
+ psi->data = kzalloc(record_size, GFP_KERNEL);
if (!psi->data)
return -ENOMEM;
@@ -52,7 +54,7 @@ static inline u64 generic_id(u64 timestamp, unsigned int part, int count)
static int efi_pstore_read_func(struct pstore_record *record,
efi_char16_t *varname)
{
- unsigned long wlen, size = EFIVARS_DATA_SIZE_MAX;
+ unsigned long wlen, size = record_size;
char name[DUMP_NAME_LEN], data_type;
efi_status_t status;
int cnt;
@@ -133,7 +135,7 @@ static ssize_t efi_pstore_read(struct pstore_record *record)
efi_status_t status;
for (;;) {
- varname_size = EFIVARS_DATA_SIZE_MAX;
+ varname_size = record_size;
/*
* If this is the first read() call in the pstore enumeration,
@@ -224,11 +226,20 @@ static __init int efivars_pstore_init(void)
if (efivars_pstore_disable)
return 0;
- efi_pstore_info.buf = kmalloc(4096, GFP_KERNEL);
+ /*
+ * Notice that 1024 is the minimum here to prevent issues with
+ * decompression algorithms that were spotted during tests;
+ * even in the case of not using compression, smaller values would
+ * just pollute more the pstore FS with many small collected files.
+ */
+ if (record_size < 1024)
+ record_size = 1024;
+
+ efi_pstore_info.buf = kmalloc(record_size, GFP_KERNEL);
if (!efi_pstore_info.buf)
return -ENOMEM;
- efi_pstore_info.bufsize = 1024;
+ efi_pstore_info.bufsize = record_size;
if (pstore_register(&efi_pstore_info)) {
kfree(efi_pstore_info.buf);
--
2.38.0On Thu, Oct 13, 2022 at 06:06:48PM -0300, Guilherme G. Piccoli wrote: > By default, the efi-pstore backend hardcode the UEFI variable size > as 1024 bytes. The historical reasons for that were discussed by > Ard in threads [0][1]: > > "there is some cargo cult from prehistoric EFI times going > on here, it seems. Or maybe just misinterpretation of the maximum > size for the variable *name* vs the variable itself.". > > "OVMF has > OvmfPkg/OvmfPkgX64.dsc: > gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x2000 > OvmfPkg/OvmfPkgX64.dsc: > gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x8400 > > where the first one is without secure boot and the second with secure > boot. Interestingly, the default is > > gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x400 > > so this is probably where this 1k number comes from." > > With that, and since there is not such a limit in the UEFI spec, we > have the confidence to hereby add a module parameter to enable advanced > users to change the UEFI record size for efi-pstore data collection, > this way allowing a much easier reading of the collected log, which is > not scattered anymore among many small files. > > Through empirical analysis we observed that extreme low values (like 8 > bytes) could eventually cause writing issues, so given that and the OVMF > default discussed, we limited the minimum value to 1024 bytes, which also > is still the default. > > [0] https://lore.kernel.org/lkml/CAMj1kXF4UyRMh2Y_KakeNBHvkHhTtavASTAxXinDO1rhPe_wYg@mail.gmail.com/ > [1] https://lore.kernel.org/lkml/CAMj1kXFy-2KddGu+dgebAdU9v2sindxVoiHLWuVhqYw+R=kqng@mail.gmail.com/ > > Cc: Ard Biesheuvel <ardb@kernel.org> > Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com> With the var length change recommended by Ard, yeah, looks good to me. :) Thanks! -Kees -- Kees Cook
On Thu, 13 Oct 2022 at 23:11, Guilherme G. Piccoli <gpiccoli@igalia.com> wrote:
>
> By default, the efi-pstore backend hardcode the UEFI variable size
> as 1024 bytes. The historical reasons for that were discussed by
> Ard in threads [0][1]:
>
> "there is some cargo cult from prehistoric EFI times going
> on here, it seems. Or maybe just misinterpretation of the maximum
> size for the variable *name* vs the variable itself.".
>
> "OVMF has
> OvmfPkg/OvmfPkgX64.dsc:
> gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x2000
> OvmfPkg/OvmfPkgX64.dsc:
> gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x8400
>
> where the first one is without secure boot and the second with secure
> boot. Interestingly, the default is
>
> gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x400
>
> so this is probably where this 1k number comes from."
>
> With that, and since there is not such a limit in the UEFI spec, we
> have the confidence to hereby add a module parameter to enable advanced
> users to change the UEFI record size for efi-pstore data collection,
> this way allowing a much easier reading of the collected log, which is
> not scattered anymore among many small files.
>
> Through empirical analysis we observed that extreme low values (like 8
> bytes) could eventually cause writing issues, so given that and the OVMF
> default discussed, we limited the minimum value to 1024 bytes, which also
> is still the default.
>
> [0] https://lore.kernel.org/lkml/CAMj1kXF4UyRMh2Y_KakeNBHvkHhTtavASTAxXinDO1rhPe_wYg@mail.gmail.com/
> [1] https://lore.kernel.org/lkml/CAMj1kXFy-2KddGu+dgebAdU9v2sindxVoiHLWuVhqYw+R=kqng@mail.gmail.com/
>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
> ---
>
>
> V2:
> - Fixed a memory corruption bug in the code (that wasn't causing
> trouble before due to the fixed sized of record_size), thanks
> Ard for spotting this!
>
> - Added Ard's archeology in the commit message plus a comment
> with the reasoning behind the minimum value.
>
>
> drivers/firmware/efi/efi-pstore.c | 23 +++++++++++++++++------
> 1 file changed, 17 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/firmware/efi/efi-pstore.c b/drivers/firmware/efi/efi-pstore.c
> index 97a9e84840a0..827e32427ddb 100644
> --- a/drivers/firmware/efi/efi-pstore.c
> +++ b/drivers/firmware/efi/efi-pstore.c
> @@ -10,7 +10,9 @@ MODULE_IMPORT_NS(EFIVAR);
>
> #define DUMP_NAME_LEN 66
>
> -#define EFIVARS_DATA_SIZE_MAX 1024
> +static unsigned int record_size = 1024;
> +module_param(record_size, uint, 0444);
> +MODULE_PARM_DESC(record_size, "size of each pstore UEFI var (in bytes, min/default=1024)");
>
> static bool efivars_pstore_disable =
> IS_ENABLED(CONFIG_EFI_VARS_PSTORE_DEFAULT_DISABLE);
> @@ -30,7 +32,7 @@ static int efi_pstore_open(struct pstore_info *psi)
> if (err)
> return err;
>
> - psi->data = kzalloc(EFIVARS_DATA_SIZE_MAX, GFP_KERNEL);
> + psi->data = kzalloc(record_size, GFP_KERNEL);
> if (!psi->data)
> return -ENOMEM;
>
> @@ -52,7 +54,7 @@ static inline u64 generic_id(u64 timestamp, unsigned int part, int count)
> static int efi_pstore_read_func(struct pstore_record *record,
> efi_char16_t *varname)
> {
> - unsigned long wlen, size = EFIVARS_DATA_SIZE_MAX;
> + unsigned long wlen, size = record_size;
> char name[DUMP_NAME_LEN], data_type;
> efi_status_t status;
> int cnt;
> @@ -133,7 +135,7 @@ static ssize_t efi_pstore_read(struct pstore_record *record)
> efi_status_t status;
>
> for (;;) {
> - varname_size = EFIVARS_DATA_SIZE_MAX;
> + varname_size = record_size;
>
I don't think we need this - this is the size of the variable name not
the variable itself.
> /*
> * If this is the first read() call in the pstore enumeration,
> @@ -224,11 +226,20 @@ static __init int efivars_pstore_init(void)
> if (efivars_pstore_disable)
> return 0;
>
> - efi_pstore_info.buf = kmalloc(4096, GFP_KERNEL);
> + /*
> + * Notice that 1024 is the minimum here to prevent issues with
> + * decompression algorithms that were spotted during tests;
> + * even in the case of not using compression, smaller values would
> + * just pollute more the pstore FS with many small collected files.
> + */
> + if (record_size < 1024)
> + record_size = 1024;
> +
> + efi_pstore_info.buf = kmalloc(record_size, GFP_KERNEL);
> if (!efi_pstore_info.buf)
> return -ENOMEM;
>
> - efi_pstore_info.bufsize = 1024;
> + efi_pstore_info.bufsize = record_size;
>
> if (pstore_register(&efi_pstore_info)) {
> kfree(efi_pstore_info.buf);
> --
> 2.38.0
>
On 14/10/2022 11:46, Ard Biesheuvel wrote:
> [...]
>> for (;;) {
>> - varname_size = EFIVARS_DATA_SIZE_MAX;
>> + varname_size = record_size;
>>
>
> I don't think we need this - this is the size of the variable name not
> the variable itself.
>
Ugh, my bad. Do you want to stick with 1024 then?
Thanks,
Guilherme
On Fri, 14 Oct 2022 at 16:58, Guilherme G. Piccoli <gpiccoli@igalia.com> wrote:
>
> On 14/10/2022 11:46, Ard Biesheuvel wrote:
> > [...]
> >> for (;;) {
> >> - varname_size = EFIVARS_DATA_SIZE_MAX;
> >> + varname_size = record_size;
> >>
> >
> > I don't think we need this - this is the size of the variable name not
> > the variable itself.
> >
>
> Ugh, my bad. Do you want to stick with 1024 then?
Yes let's keep this at 1024
On 14/10/2022 12:00, Ard Biesheuvel wrote:
> On Fri, 14 Oct 2022 at 16:58, Guilherme G. Piccoli <gpiccoli@igalia.com> wrote:
>>
>> On 14/10/2022 11:46, Ard Biesheuvel wrote:
>>> [...]
>>>> for (;;) {
>>>> - varname_size = EFIVARS_DATA_SIZE_MAX;
>>>> + varname_size = record_size;
>>>>
>>>
>>> I don't think we need this - this is the size of the variable name not
>>> the variable itself.
>>>
>>
>> Ugh, my bad. Do you want to stick with 1024 then?
>
> Yes let's keep this at 1024
Perfect, will re-send after we have more feedback on patches 1 and 2.
Thanks again,
Guilherme
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