Validating Signature Of XPC Process

Question

Quinn, you've often suggested that to validate the other side of an XPC connection, we should use the audit token. But that's not available from the XPC object, whereas the PID is. So everyone uses the PID.

While looking for something completely unrelated, I found this in the SecCode.h file

OSStatus SecCodeCreateWithXPCMessage(xpc_object_t message, SecCSFlags flags,
	SecCodeRef * __nonnull CF_RETURNS_RETAINED target);

Would this be the preferred way to do this now? At least from 11.0 and up.

Like I said, I was looking for something completely unrelated and found this and don't have the cycles right now to try it. But it looks promising from the description and I wanted to check in with you about it in case you can say yes or no before I get a chance to test it.

Thanks

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Posted by

mdolan

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Answer 1

I’m posting this summary so that I can mark it as Apple Recommended and (finally!) but this issue to bed. So:

If you’re using NSXPCConnection, use -setCodeSigningRequirement: method to solve this problem. This was introduced with macOS 13 (currently in beta).
If you’re using the XPC C API, use the xpc_connection_set_peer_code_signing_requirement API to solve this problem. This was introduced with macOS 12.

Both of these APIs take a code signing requirement. For more information on that topic, see TN3127 Inside Code Signing: Requirements.
If you’re using the C API on macOS 11, use SecCodeCreateWithXPCMessage for this. There’s currently no documentation but there are good doc comments in <Security/SecCode.h>.
If none of the above apply, there isn’t a great solution. Various options have been discussed here, including on the older thread referenced above, but there’s nothing I’m fully confident in.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Posted by

eskimo

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Answer 2

So everyone uses the PID.

I thought everyone just using the non-public stuff (-;

Would this be the preferred way to do this now?

Absolutely.

The main drawback here is that this is tied to the XPC C API, and thus isn’t available to folks using NSXPCConnection (r. 27605275).

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Posted by

eskimo

What? Use non-public stuff? Never!

Since the code I have is C, that is absolutely not a problem.

Thanks for the quick response.

—
mdolan
So.... if that call isn't available to us mere NSXPCConnection users, what could we do instead? I'm verifyng caller at the

- (BOOL)listener:(NSXPCListener *)listener shouldAcceptNewConnection:(NSXPCConnection *)newConnection;

level, identifying the caller by PID, but every once in a while

NSDictionary *attributes = @{(__bridge NSString *)kSecGuestAttributePid : @(clientPID) }; OSStatus status = SecCodeCopyGuestWithAttributes(NULL, (__bridge CFDictionaryRef)attributes, kSecCSDefaultFlags, &dynamicCode);

crashes my XPC Service, with nice stack somewhere deep in SecCodeCopyGuestWithAttributes:

` 0 libsystem_kernel.dylib 0x183dcd9b8 __pthread_kill + 8
1 libsystem_pthread.dylib 0x183e0115c pthread_kill + 288 2 libsystem_c.dylib 0x183d3e314 abort + 164 3 libsystem_malloc.dylib 0x183c23a1c malloc_vreport + 552 4 libsystem_malloc.dylib 0x183c38c8c malloc_zone_error + 104 5 libsystem_malloc.dylib 0x183c15db0 nanov2_allocate_from_block + 568 6 libsystem_malloc.dylib 0x183c153a4 nanov2_allocate + 128 7 libsystem_malloc.dylib 0x183c152c0 nanov2_malloc + 64 8 libsystem_malloc.dylib 0x183c32770 _malloc_zone_malloc + 156 9 CoreFoundation 0x183e5ab0c resolveAbsoluteURLStringBuffer + 1012
10 CoreFoundation 0x183e5a678 resolveAbsoluteURLString + 188

11 CoreFoundation 0x183e58744 CFURLCopyAbsoluteURL + 568

12 CoreFoundation 0x183f6f750 _CFURLCreateWithFileSystemPath + 2236

13 CoreFoundation 0x183eb874c _CFBundleCopyExecutableURLRaw + 320

14 CoreFoundation 0x183eb84e0 _CFBundleCopyExecutableURLInDirectory2 + 452 15 CoreFoundation 0x183f37ff0 _CFBundleCreateWithExecutableURLIfLooksLikeBundle + 128 16 CoreFoundation 0x183f37f24 _CFBundleCreateWithExecutableURLIfMightBeBundle + 20 17 Security 0x1860d3d18 Security::CodeSigning::KernelCode::identifyGuest(Security::CodeSigning::SecCode*, __CFData const**) + 544 18 Security 0x1860ab040 Security::CodeSigning::SecCode::identify() + 96 19 Security 0x1860ab8c0 Security::CodeSigning::SecCode::autoLocateGuest(__CFDictionary const*, unsigned int) + 188 20 Security 0x1860b2318 SecCodeCopyGuestWithAttributes + 144 21 xpcj 0x11706c3b0 -[OITContentScanningXPCService listener:shouldAcceptNewConnection:] + 556 (OITContentScanningXPCService.m:209) 22 Foundation 0x184e274c8 `

So... how to go about this, and is it better to use the kSecGuestAttributeAudit instead of the kSecGuestAttributePid when calling SecCodeCopyGuestWithAttributes ?

—
suMac
I can't read this; please put it in a reply. Or just start a new thread and reference this one.

—
eskimo

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Answer 3

I’m very please to announce that macOS 12 beta includes a new API that represents a great solution to this problem: Check out xpc_connection_set_peer_code_signing_requirement, and its associated doc comments, in <xpc/connection.h>.

Still no news on the NSXPCConnection side of this (r. 27605275)-:

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Posted by

eskimo

This looks great!! 🎉 FWIW, I filed FB9209390 asking for an NSXPCConnection equivalent.

—
Ryder
Ta!

—
eskimo

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Answer 4

I’m posting this link to a much older thread discussing this issue, just for context.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Posted by

eskimo

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Answer 5

I needed a secure XPC Mach Services connection and didn't want to be directly using the XPC C API throughout my codebase, so I created the SecureXPC framework for Swift. It uses the aforementioned SecCodeCreateWithXPCMessage API on macOS 11 and later, and falls back to as eskimo says the "non-public stuff" on older versions. Sharing this in the hopes it's helpful to people here. Feedback most welcome over on the Github page either as issues or discussions!

Posted by

jkaplan

Thanks for sharing!

—
eskimo

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Answer 6

I've just started using xpc_connection_set_peer_code_signing_requirement() and can happily report that it meets all of my needs in terms of validating who my XPC connection is really connected to. However there seems to have been a slight oversight in that the new error XPC_ERROR_PEER_CODE_SIGNING_REQUIREMENT has either not been made public, or is not available to Swift code for some reason. For example:

if event === XPC_ERROR_CONNECTION_INVALID { // OK
} else if event === XPC_ERROR_TERMINATION_IMMINENT { // OK
} else if event === XPC_ERROR_CONNECTION_INTERRUPTED { // OK 
} else if event === XPC_ERROR_PEER_CODE_SIGNING_REQUIREMENT { // Error: Cannot find in scope
}

Posted by

fractal

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Answer 7

has either not been made public

It’s definitely public, declared in <xpc/connection.h> in the macOS 12.0 SDK.

or is not available to Swift code for some reason

Indeed.

XPC_ERROR_PEER_CODE_SIGNING_REQUIREMENT is actually a macro and it seems likely that this macro is structured in a way that prevents the Swift importer from seeing it. Please file a bug about that, then post your bug number, just for the record.

Working around this is relatively straightforward: Define a C function that returns XPC_ERROR_PEER_CODE_SIGNING_REQUIREMENT and call that from your Swift code.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Posted by

eskimo

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Answer 8

Earlier I wrote:

Still no news on the NSXPCConnection side of this (r. 27605275)-:

I’m very pleased to say that macOS 13 beta has a shiny new -[NSXPCConnection setCodeSigningRequirement:] method. It even comes with documentation! Please take it for a spin and let us know if you hit any problems.

For an in-depth discussion of code signing requirements, see TN3127 Inside Code Signing: Requirements.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Posted by

eskimo

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Answer 9

I’m posting this summary so that I can mark it as Apple Recommended and (finally!) but this issue to bed. So:

If you’re using NSXPCConnection, use -setCodeSigningRequirement: method to solve this problem. This was introduced with macOS 13 (currently in beta).
If you’re using the XPC C API, use the xpc_connection_set_peer_code_signing_requirement API to solve this problem. This was introduced with macOS 12.

Both of these APIs take a code signing requirement. For more information on that topic, see TN3127 Inside Code Signing: Requirements.
If you’re using the C API on macOS 11, use SecCodeCreateWithXPCMessage for this. There’s currently no documentation but there are good doc comments in <Security/SecCode.h>.
If none of the above apply, there isn’t a great solution. Various options have been discussed here, including on the older thread referenced above, but there’s nothing I’m fully confident in.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Posted by

eskimo

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Validating Signature Of XPC Process

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